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Accurizing is the process of improving the
accuracy and precision Accuracy and precision are two measures of ''observational error''. ''Accuracy'' is how close a given set of measurements ( observations or readings) are to their '' true value'', while ''precision'' is how close the measurements are to each ot ...
of a gun (
firearm A firearm is any type of gun designed to be readily carried and used by an individual. The term is legally defined further in different countries (see Legal definitions). The first firearms originated in 10th-century China, when bamboo tubes ...
or
airgun An air gun or airgun is a gun that fires projectiles pneumatically with compressed air or other gases that are mechanically pressurized ''without'' involving any chemical reactions, in contrast to a firearm, which pressurizes gases ''c ...
). For firearms,
accuracy Accuracy and precision are two measures of '' observational error''. ''Accuracy'' is how close a given set of measurements ( observations or readings) are to their '' true value'', while ''precision'' is how close the measurements are to each o ...
is the ability to hit exactly what one is aiming at, and
precision Precision, precise or precisely may refer to: Science, and technology, and mathematics Mathematics and computing (general) * Accuracy and precision, measurement deviation from true value and its scatter * Significant figures, the number of digit ...
is the ability to hit the same place over and over again in a repeatable fashion. Both are the goals of accurizing, which generally concentrates on four different areas: *Usability: Enhancements that give the shooter a firmer and more controlled hold on the firearm, as well as a more consistent trigger pull. Better design
ergonomics Human factors and ergonomics (commonly referred to as human factors) is the application of psychological and physiological principles to the engineering and design of products, processes, and systems. Four primary goals of human factors learnin ...
is often employed, such as adjustable
buttstock A gunstock or often simply stock, the back portion of which is also known as a shoulder stock, a buttstock or simply a butt, is a part of a long gun that provides structural support, to which the barrel, action, and firing mechanism are attache ...
s and grips with more vertical angles that are natural to the human hand and wrist (e.g.
pistol grip On a firearm or other tools, a pistol grip is a distinctly protruded handle underneath the main mechanism, to be held by the user's hand at a more vertical (and thus more ergonomic) angle, similar to the how one would hold a conventional pis ...
).
Spirit level A spirit level, bubble level, or simply a level, is an instrument designed to indicate whether a surface is horizontal (level) or vertical ( plumb). Different types of spirit levels may be used by carpenters, stonemasons, bricklayers, ...
s are often mounted to prevent
cant Cant, CANT, canting, or canted may refer to: Language * Cant (language), a secret language * Beurla Reagaird, a language of the Scottish Highland Travellers * Scottish Cant, a language of the Scottish Lowland Travellers * Shelta or the Cant, a la ...
ing, which can vary the points of impact.
Weapon mount A weapon mount is an assembly or mechanism used to hold a weapon (typically a gun) onto a platform in order for it to function at maximum capacity. Weapon mounts can be broken down into two categories: static mounts and non-static mounts. Sta ...
s such as
bipod A bipod is a V-shaped portable attachment that helps support and steady a device, usually a weapon such as a long gun or a mortar. The term comes from the Latin prefix ''bi-'' and Greek root ''pod'', meaning "two" and "foot" respectively. Bip ...
s,
monopod A monopod, also called a unipod, is a single staff or pole used to help support cameras, binoculars, rifles or other precision instruments in the field. Camera and imaging use The monopod allows a still camera to be held steadier, allowing t ...
s, benchrests,
shooting sticks Shooting sticks are portable weapon mounts commonly used with rifles to brace the weapon on the ground, walls, and other features of the local terrain in order to provide a stable resting position to shoot from; reducing user fatigue and increa ...
or simply
sandbag A sandbag or dirtbag is a bag or sack made of hessian (burlap), polypropylene or other sturdy materials that is filled with sand or soil and used for such purposes as flood control, military fortification in trenches and bunkers, shielding gl ...
s can provide a more stable and relaxed platform for the shooter, and devices such as
muzzle brake A muzzle brake or recoil compensator is a device connected to, or a feature integral to the construction of, the muzzle or barrel of a firearm or cannon that is intended to redirect a portion of propellant gases to counter recoil and unwanted ...
s or compensators can also be used to help counter the
muzzle rise Muzzle rise, muzzle flip or muzzle climb refers to the tendency of a firearm's or airgun's Muzzle (firearms), muzzle (front end of the Gun barrel, barrel) to rise up after firing. It more specifically refers to the seemingly unpredictable "jump" ...
from
recoil Recoil (often called knockback, kickback or simply kick) is the rearward thrust generated when a gun is being discharged. In technical terms, the recoil is a result of conservation of momentum, as according to Newton's third law the force r ...
and re-establish aim faster and more precisely for repeated firing. The use of suitable
sling sling may refer to: Places * Sling, Anglesey, Wales * Sling, Gloucestershire, England, a small village in the Forest of Dean People with the name * Otto Šling (1912–1952), repressed Czech communist functionary Arts, entertainment, and media ...
s can also help shooters to stabilize their aim when shooting off-hand while standing or squatting. *Tolerances: Parts that better fit together will shift less, or shift more consistently, under recoil.
Rifle bedding Rifle bedding is a process of providing a rigid and consistent foundation for a rifle’s operational components, by creating a stable and close-fitting bearing surface between the gun’s functional parts (i.e. the receiver housing the barrelle ...
is one of the most common practices of such accurizing procedure. Adequate screw
torque In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of th ...
setting between the
action Action may refer to: * Action (narrative), a literary mode * Action fiction, a type of genre fiction * Action game, a genre of video game Film * Action film, a genre of film * ''Action'' (1921 film), a film by John Ford * ''Action'' (1980 fil ...
and the
stock In finance, stock (also capital stock) consists of all the shares by which ownership of a corporation or company is divided.Longman Business English Dictionary: "stock - ''especially AmE'' one of the shares into which ownership of a compan ...
is also important for the overall rigidity of the system. Some companies, such as
Savage Arms Savage Arms is an American gunmaker based in Westfield, Massachusetts, with operations in Canada. Savage makes a variety of rimfire and centerfire rifles, as well as Stevens single-shot rifles and shotguns. The company is best known for the ...
, have even introduced features like floating
bolt head Bolt Head is a National Trust headland on the South Coast of Devon, England, United Kingdom, situated west of the Kingsbridge Estuary. ...
to provide better bolt-breech engagement for more adequate breech
seal Seal may refer to any of the following: Common uses * Pinniped, a diverse group of semi-aquatic marine mammals, many of which are commonly called seals, particularly: ** Earless seal, or "true seal" ** Fur seal * Seal (emblem), a device to imp ...
and headspacing. *Harmonics: The act of firing a gun generates a rapid pressure increase within the barrel bore, causing the barrel to
resonate Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillat ...
and
vibrate Vibration is a mechanical phenomenon whereby oscillations occur about an equilibrium point. The word comes from Latin ''vibrationem'' ("shaking, brandishing"). The oscillations may be periodic function, periodic, such as the motion of a pendulum ...
in a rope-like fashion. The resultant
harmonic oscillation In classical mechanics, a harmonic oscillator is a system that, when displaced from its equilibrium position, experiences a restoring force ''F'' proportional to the displacement ''x'': \vec F = -k \vec x, where ''k'' is a positive consta ...
s of the barrel affect the terminal phase of the projectile's
internal ballistics Internal ballistics (also interior ballistics), a subfield of ballistics, is the study of the propulsion of a projectile. In guns, internal ballistics covers the time from the propellant's ignition until the projectile exits the gun barrel. The s ...
and in turn the initial status of its
external ballistics External ballistics or exterior ballistics is the part of ballistics that deals with the behavior of a projectile in flight. The projectile may be powered or un-powered, guided or unguided, spin or fin stabilized, flying through an atmosphere o ...
, and therefore need to be minimized or tuned to limit their effects on accuracy. Generally the harmonic effects are proportional to the square of the barrel length, and so are generally only of concern in
long gun A long gun is a category of firearms with long barrels. In small arms, a ''long gun'' or longarm is generally designed to be held by both hands and braced against the shoulder, in contrast to a handgun, which can be fired being held with a single ...
s such as
rifle A rifle is a long-barreled firearm designed for accurate shooting, with a barrel that has a helical pattern of grooves ( rifling) cut into the bore wall. In keeping with their focus on accuracy, rifles are typically designed to be held with ...
s but not
handgun A handgun is a short-barrelled gun, typically a firearm, that is designed to be usable with only one hand. It is distinguished from a long gun (i.e. rifle, shotgun or machine gun, etc.), which needs to be held by both hands and also braced ag ...
s. Some external accessories, called ''tuners'' or ''de-resonators'', can also be mounted onto the barrel to alter the harmonic wave pattern so that the
node In general, a node is a localized swelling (a " knot") or a point of intersection (a vertex). Node may refer to: In mathematics * Vertex (graph theory), a vertex in a mathematical graph * Vertex (geometry), a point where two or more curves, line ...
is shifted as near to the muzzle as possible. Airguns have significantly lower barrel pressure, and therefore are far less affected by barrel harmonics than firearms. *Projectile propulsion consistency: In airguns, the inbuilt powerplants themselves provide the propulsive force to the
projectile A projectile is an object that is propelled by the application of an external force and then moves freely under the influence of gravity and air resistance. Although any objects in motion through space are projectiles, they are commonly found i ...
, so tuning the gun alone is usually sufficient for accurizing as long as the projectiles' weights and shapes are uniform. Firearms, however, rely purely on
oxidative Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking ...
of the
powder A powder is a dry, bulk solid composed of many very fine particles that may flow freely when shaken or tilted. Powders are a special sub-class of granular materials, although the terms ''powder'' and '' granular'' are sometimes used to distin ...
within a
cartridge Cartridge may refer to: Objects * Cartridge (firearms), a type of modern ammunition * ROM cartridge, a removable component in an electronic device * Cartridge (respirator), a type of filter used in respirators Other uses * Cartridge (surname), a ...
to provide propulsive force, and any slight variations in powder load and combustion efficiency will affect the
internal ballistics Internal ballistics (also interior ballistics), a subfield of ballistics, is the study of the propulsion of a projectile. In guns, internal ballistics covers the time from the propellant's ignition until the projectile exits the gun barrel. The s ...
of the gun, even if the projectile weights and shape are the same. This means that in addition to the gun itself, consistent ammunition performance is also extremely critical for accuracy with firearms. While some manufacturers produce match-grade ammunition with smaller
tolerance Tolerance or toleration is the state of tolerating, or putting up with, conditionally. Economics, business, and politics * Toleration Party, a historic political party active in Connecticut * Tolerant Systems, the former name of Veritas Software ...
s, it is common for shooters of high-precision disciplines to
handload Handloading, or reloading, is the practice of making firearm cartridges by assembling the individual components (case, primer, propellant, and projectile), rather than purchasing mass-assembled, factory-loaded ammunition. The term ''handlo ...
and fine-tune their own ammunition. Furthermore, the rapid gas expansion that occurs when the projectile leaves the muzzle also barometrically affects flight behaviour, so muzzle devices such as
flash hider A flash suppressor, also known as a flash guard, flash eliminator, flash hider, or flash cone, is a muzzle device attached to the muzzle of a rifle that reduces its visible signature while firing by cooling or dispersing the burning gases that ...
and
suppressor A silencer, also known as a sound suppressor, suppressor, or sound moderator, is a muzzle device that reduces the acoustic intensity of the muzzle report (sound of a gunshot) and muzzle rise when a gun (firearm or air gun) is discharged, b ...
can also be used to modulate the escaping gas and improve the consistency of shots. The key to an accurate firearm is consistency. Getting everything to happen the same way for every shot is key to producing small
grouping Grouping may refer to: * Muenchian grouping * Principles of grouping * Railways Act 1921, also known as Grouping Act, a reorganisation of the British railway system * Grouping (firearms), the pattern of multiple shots from a sidearm See also ...
s, and there are a large number of issues to be addressed in achieving an accurate firearm. The keys to firing an accurate shot are a firm but not overtight grip, the ability to get a good sight picture and a controlled squeeze of the trigger. The ability to manage recoil is also important in heavily recoiling calibers, both to aid in possible additional shots, and to prevent the user from developing a fear of the recoil.


Determining accuracy

Determining accuracy is not always a straightforward task, as it depends on a large number of variables.


Factors affecting accuracy

The accuracy of a shot relies on many different factors, which can be broken down into three broad categories: the firearm, the cartridge, and the shooter. Accurizing generally refers to the processes that are applied to the firearm. Techniques relating to producing accurate ammunition are covered in
internal Internal may refer to: *Internality as a concept in behavioural economics *Neijia, internal styles of Chinese martial arts *Neigong or "internal skills", a type of exercise in meditation associated with Daoism *''Internal (album)'' by Safia, 2016 ...
and
external ballistics External ballistics or exterior ballistics is the part of ballistics that deals with the behavior of a projectile in flight. The projectile may be powered or un-powered, guided or unguided, spin or fin stabilized, flying through an atmosphere o ...
, and
handloading Handloading, or reloading, is the practice of making firearm cartridges by assembling the individual components (case, primer, propellant, and projectile), rather than purchasing mass-assembled, factory-loaded ammunition. The term ''handlo ...
, and just like accurizing a firearm, the goal is to produce the most consistent possible results. The shooter must also be consistent, and this means that the fundamentals of
marksmanship A marksman is a person who is skilled in precision shooting using projectile weapons (in modern days most commonly an accurized scoped long gun such as designated marksman rifle or a sniper rifle) to shoot at high-value targets at longer-th ...
have to be followed rigorously; any failure on the part of the shooter to remain focused and consistent can result in a bad shot. It's common to use a benchrest or a vise when evaluating ammunition or a weapon for accuracy in order to eliminate human error.


Measurements

Since adjusting the point of impact to match the point of aim is relatively simple with any type of adjustable sights, the primary goal of accurizing is to increase the precision of the firearm, which is generally measured by looking at the dispersion of a number of shots fired at the same point of aim. An ideal group would be one where all shots land in a hole no larger than the diameter of a single bullet; this would indicate zero dispersion. The most common way of measuring groups then is to measure the edge to edge distance of the farthest holes, and subtract the bullet diameter, which gives the ''center to center'' or ''c-c'' measurement of the group. This can be expressed in linear measures (''a 30 mm group at 100 m'', or ''a one inch group at 100 yards'') or in angular measures (''a
milliradian A milliradian ( SI-symbol mrad, sometimes also abbreviated mil) is an SI derived unit for angular measurement which is defined as a thousandth of a radian (0.001 radian). Milliradians are used in adjustment of firearm sights by adjusting t ...
'' or ''
MOA Moa are extinct giant flightless birds native to New Zealand. The term has also come to be used for chicken in many Polynesian cultures and is found in the names of many chicken recipes, such as Kale moa and Moa Samoa. Moa or MOA may also refe ...
group''). Groups for rifles are traditionally shot at either 100 meters or . At 100 yd a minute of arc equals , and the one MOA group (approximately 1/3 or 0.3 mil) is a traditional
benchmark Benchmark may refer to: Business and economics * Benchmarking, evaluating performance within organizations * Benchmark price * Benchmark (crude oil), oil-specific practices Science and technology * Benchmark (surveying), a point of known elevati ...
of accuracy. Handguns are generally used at closer ranges, and are tested for accuracy at their intended range of use. Also of importance is the number of shots fired. Statistical likelihood says the fewer shots that are fired, the smaller the dispersion will be. 3 or 5-shot groups are acceptable for zeroing the sights and rough accuracy estimates, but most shooters consider 10-shot groups to be the minimum for accuracy comparisons.


Defining accuracy

Even defining accuracy can be problematic. An example of this can be shown by the following tests, run by ''Performance Shooter'' magazine in December, 1996. The magazine was testing seven brands of
.38 Special The .38 Special, also commonly known as .38 S&W Special (not to be confused with .38 S&W), .38 Smith & Wesson Special, .38 Spl, .38 Spc, (pronounced "thirty-eight special"), or 9x29mmR is a rimmed, centerfire cartridge designed by Smith & ...
wadcutter rounds in three different
revolver A revolver (also called a wheel gun) is a repeating firearm, repeating handgun that has at least one gun barrel, barrel and uses a revolving cylinder (firearms), cylinder containing multiple chamber (firearms), chambers (each holding a single ...
s, a
Smith & Wesson Model 686 The Smith & Wesson Model 686 is a six- or seven-shot double-action revolver manufactured by Smith & Wesson and chambered for the .357 Magnum cartridge; it will also chamber and fire .38 Special cartridges. Smith & Wesson introduced the Model 686 ...
and Model 52, and a
Colt Python The Colt Python is a .357 Magnum caliber revolver manufactured by Colt's Manufacturing Company of Hartford, Connecticut.Dougherty, Martin ''Small Arms: From the Civil War to the Present Day'', New York City: Fall River Press, 2005, page 48. It w ...
Target model, with six, five and eight inch long barrels, respectively. Ten groups of five shots were fired and measured from each revolver with each ammunition. Click on the image at right to see a larger view of the graph of average group sizes for each type of ammunition and each revolver. The average group size for the overall test was . Based on
average In ordinary language, an average is a single number taken as representative of a list of numbers, usually the sum of the numbers divided by how many numbers are in the list (the arithmetic mean). For example, the average of the numbers 2, 3, 4, 7 ...
group size, the winner was the Model 686, which shot an average group of across the brands of ammunition, with a standard deviation between ammunition types of . However, the Model 52, while shooting slightly larger groups at , was far more consistent across the brands, with a
standard deviation In statistics, the standard deviation is a measure of the amount of variation or dispersion of a set of values. A low standard deviation indicates that the values tend to be close to the mean (also called the expected value) of the set, whil ...
of only , and was the most consistent performer of the test. However, if the ammunition was tuned to the gun, the clear winner was the Python, which averaged just with its favored brand of ammunition. The Python was also by far the pickiest, however, turning in the largest groups at averages with its least favorite brands, for a standard deviation of . Based on this test, answering the question "Which is the most accurate?" becomes a matter of opinion. The 686 shot the best average groups. However, as the Python showed the best performance with one brand of ammunition, it might be the best choice if that brand of ammunition were acceptable for the application in question. If a consistent supply of ammunition were a problem, then the 52 might be the best choice, since it showed the least sensitivity to differences in ammunition.


Testing methodology

As the goal of accurizing a firearm is to improve its accuracy, the way accuracy is measured becomes important. A firearm used primarily as a
hunting weapon Hunting weapons are weapons designed or used primarily for hunting game animals for food or sport, as distinct from defensive weapons or weapons used primarily in warfare. Characteristics Since human beings are lacking in the natural weapons pos ...
will need to be accurate on the first shot from a cold, clean barrel, while one used for target shooting may be allowed fouling shots before the first shot for record is fired. Issues of portability or restrictions of certain competitions may limit the alterations that can be made. In addition, every firearm is different, and processes that yield good results on one may not affect another. Another issue in measuring accuracy is the method by which the gun is secured for the test. The most accurate shooting position is a supported position, such as firing from a benchrest with the firearm well-supported by a shooting rest or sandbags; this eliminates much of the shooter's potential for error and will generally result in much smaller groups than firing from an unsupported position. Even for a firearm that is going to be shot offhand, accuracy testing from a machine rest will provide an idea of the ultimate attainable accuracy.


Usability

No matter what the potential laboratory accuracy of a firearm, it does not matter if a human shooter cannot fire it accurately under a set of real-world conditions. A firearm that is comfortable, fits the user well, and provides for a careful, consistent trigger pull and recoil management is not only a physical advantage over a poorly fit firearm, but a psychological one as well.


Trigger

The dynamics of the trigger are one of the most important aspects of usability, since any movement of the firearm caused by pulling the trigger can affect the placement of the shot. Trigger pulls are relative, however. Compare a sport like
action shooting Practical shooting, also known as dynamic shooting or action shooting, is a set of shooting sports where the competitors try to unite the three principles of precision, power, and speed, by using a firearm of a certain minimum power factor to sco ...
, which emphasizes speed and uses relatively close targets with large scoring zones on the targets, to
bullseye Bullseye or Bull's Eye may refer to: Symbols * ◎ (Unicode U+25CE BULLSEYE), in the Geometric Shapes Unicode block * (Unicode U+0298 LATIN LETTER BILABIAL CLICK), the phonetic symbol for bilabial click Animals and plants * Bull's Eye, '' Eury ...
shooting, which uses distant targets with tiny scoring zones. While both types of trigger need a predictable pull, bullseye shooters demand a much higher degree of precision.


Analysis of the trigger pull

The trigger pull consists of three stages: # Takeup or pretravel, which is the movement of the trigger which happens before the
sear The Sahar Elevated Access Road, abbreviated to SEAR, is a dedicated, elevated, express access road in Mumbai that connects the Western Express Highway (WEH) near Hanuman Nagar junction in Vile Parle, with the forecourts of Terminal T2 of the C ...
moves. # Break, the movement during which the trigger moves the sear to the point of release. # Overtravel, which is the distance a trigger moves after the sear releases. The takeup is the least critical stage of the trigger pull, and individual preferences vary widely. Two-stage triggers, for example, consist of a noticeable takeup, followed by a distinct increase in the force required to pull the trigger, followed by the break. A single-stage trigger, on the other hand, has no discernible movement before the break. Fully adjustable triggers will provide a two-stage pull, and the option of reducing the first stage travel to zero, essentially making the trigger a single-stage trigger. The break is a far more critical stage of the pull, as it happens just prior to the shot being fired. Here again, individual preferences vary; some shooters prefer a ''soft'' break, where there is a smooth but discernible amount of trigger travel during firing, while others prefer a ''crisp'' break, with a heavier weight and little or no discernible movement. The overtravel can be the most critical factor in the trigger pull, as any movement caused at this point will happen as the shot is fired. This is especially important with firearms where there is a sudden release of resistance when the sear breaks, such as in
double-action Double action (or double-action) refers to one of two systems in firearms where the trigger both cocks and releases the hammer. *Double-action only (DAO) firearms trigger: The trigger both cocks and releases the hammer. There is no single-action f ...
triggers. An overtravel stop will arrest the motion of the trigger just after the break, and prevent movement. Over travel is not always considered bad by some, as the force of the trigger finger does not impact on the gun directly after sear release.


Improving the trigger pull

An adjustable trigger may have ways to adjust all of these stages, plus trigger location. For example, a first stage or takeup adjustment might include weight and travel, a second stage or sear engagement adjustment might include weight and travel, and a trigger stop adjustment would limit the overtravel. While adjustable triggers may provide the greatest level of control, much can be done with standard non-adjustable triggers. Careful hand fitting and polishing of parts, addition of high precision or adjustable aftermarket parts, or fabrication of new parts can greatly improve most triggers. Care should be taken, however, since trigger work requires a great deal of care and precision, and a bad trigger job can easily render a firearm highly unsafe or unusable.


Liability issues

Most manufacturers ship firearms with fairly heavy, nonadjustable triggers, colloquially known as ''lawyer triggers''. This is out of concern for liability; firearms are inherently dangerous, and allowing the user to adjust the trigger, or even implying such adjustments can be made, exposes the manufacturer to lawsuit. Likewise, manufacturers of aftermarket parts expose themselves to similar liability issues.


Sights

A firearm's sights help its user to align the barrel with an intended target. In some cases the only refinement in a "target" firearm over a standard model is improved sights. Chapter 7, You Can Build an Overnight Target Pistol Adjustable sights are essential for most target shooting, as they allow compensation for variables such as target range and ammunition type. Firearms with non-adjustable or coarsely adjustable sights cannot give their holders the ability to reliably shoot on target in changing conditions. Improvements in visibility and sharpness of a target image provided by some sights can also improve users' aim and consistency. Typical
open sights Iron sights are a system of physical alignment markers (usually made of metallic material) used as a sighting device to assist the accurate aiming of ranged weapons (such as a firearm, airgun, crossbow or even compound bow), or less commonly a ...
with a blade, post or bead near the muzzle and a notch over the breech are good for quick alignment but far from ideal for accuracy. An
aperture sight Iron sights are a system of physical alignment markers (usually made of metallic material) used as a sighting device to assist the accurate aiming of ranged weapons (such as a firearm, airgun, crossbow or even compound bow), or less commonly as ...
mounted closer to the user's eye and farther from the front sight tightens accuracy by lengthening the sight radius while helping the person take better advantage of the improvement. Some of these "peep sights" provide precise, repeatable adjustments for long range shooting with no need for tools. Telescopic and reflex sights offer advantages to people with less experience or poor eyesight by bringing both target and aiming point into focus, while "scopes" also magnify and brighten the image. Drawbacks such as weight, bulk, and complexity can also affect a shooter's performance.
Telescopic sight A telescopic sight, commonly called a scope informally, is an optical sighting device based on a refracting telescope. It is equipped with some form of a referencing pattern – known as a '' reticle'' – mounted in a focally appropriate ...
, reflex sight


Stocks and grips

A good
stock In finance, stock (also capital stock) consists of all the shares by which ownership of a corporation or company is divided.Longman Business English Dictionary: "stock - ''especially AmE'' one of the shares into which ownership of a compan ...
or grip is one that allows the shooter to have a relaxed but firm grip on the firearm. This can range from minor changes such as texturing grip surfaces or adding a wide, beavertail type grip safety to a 1911, all the way to a custom built, anatomically designed grip that "fits like a glove". Key features are: *Structure. A firearm that is properly affixed to the stock, allowing the barrel to float freely as opposed to touching other surfaces, will be more accurate. A stock should also allow the action to be affixed securely and provide a good bedding surface, which sometimes requires the use of epoxy resins. *Comfort. This allows the shooter to relax and concentrate on shooting. *Control. The shooter must be able to hold the firearm on target, and provide consistent movement under recoil. *Positioning. The shooter must be positioned correctly and consistently, allowing easy use of the sights, and a clean trigger pull.


Gripping surfaces

Gripping surfaces, particularly on handguns, are often designed to provide a large degree of friction to prevent the shooter's grip from shifting. This can be done by either using a material that provides a high friction, such as
rubber Rubber, also called India rubber, latex, Amazonian rubber, ''caucho'', or ''caoutchouc'', as initially produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds. Thailand, Malaysia, and ...
, or by adding texture to the grips. Traditionally wood grips and stocks are provided with checkering, a process where "V" shaped grooves are cut into the wood at an angle to each other, leaving a pattern of pyramidal projections in the wood. Another process uses a punch to leave a random pattern of dents in the surface, called
stippling Stippling is the creation of a pattern simulating varying degrees of solidity or shading by using small dots. Such a pattern may occur in nature and these effects are frequently emulated by artists. Art In printmaking, stipple engraving is ...
; this process is better suited to complex curves than checkering, and is often found on anatomical grips. Handguns with plastic frames will often have stippling or checkering molded into the frame.


Handgun grips

Grips for handguns, especially ones used in single handed disciplines such as bullseye and Olympic handgun shooting, are critical to good accuracy. The grip provides very little control compared to a rifle stock, and so a good fit is required to give the shooter control over the gun and isolate the trigger finger movement. Aftermarket parts makers provide a wide range of grips for handguns, allowing shooters to find grips that are suitable for their hands. Competition handgun grips are anatomically shaped, and are often custom fitted to the shooter's hand. An ideal grip will fit the contours of the shooter's grip, so that the knuckles fall into position the same way every time. The ideal grip should allow the shooter to repeatedly grasp the gun, bring it up to shooting position with eyes closed, and be correctly aligned and on target when their eyes are opened; this means the grip provides a consistent placement, and minimal corrections when aiming. Contrary to commonly stated rules of thumb, the handgun should not line up with the forearm when gripped, but rather point slightly outwards, so that when held to shoot, the sights line up with the shooter's eye. In cross-dominant shooters, the angle will be more pronounced. Thumbrests, finger grooves (if well fitted), and palm rests all provide control over recoil. An adjustable palm rest is a desirable feature as well, as it allows the grip to be adjusted to fit the shooter's hand as it swells and contracts over time.


Rifle and shotgun stocks

Stocks for long guns are not as often changed as handgun grips, but a well fitted stock can make a significant difference in accuracy. For shotguns in particular, the placement of the shooter's face on the stock provides the rear aiming point, and the correct drop, toe alignment, and cast-off can greatly enhance accuracy. This was traditionally accomplished by steaming and bending the stock, but a simpler solution for modern guns is a set of shims that alter the angle of the stock. Rifle stocks have similar issues of fit, and though the use of sights makes this less vital than in a shotgun, a good fit still helps the shooter to relax and concentrate on fundamentals. The shape of a rifle stock should be suited to its intended use. High combs and vertical pistol grips are ideal for high mounted target sights or scopes and careful, deliberate shooting such as encountered in traditional target shooting,
metallic silhouette Metallic silhouette shooting is a group of target shooting disciplines that involves shooting at steel targets representing game animals at varying distances, seeking to knock the metal target over. Metallic silhouette is shot with large bore rifl ...
, or
varmint hunting Varmint hunting or varminting is the practice of hunting vermin — generally small/medium-sized wild mammals or birds — as a means of pest control, rather than as games for food or trophy. The targeted animals are culled because they are c ...
, as they provide a maximum
point blank range Point-blank range is any distance over which a certain firearm can hit a target without the need to compensate for bullet drop, and can be adjusted over a wide range of distances by sighting in the firearm. If the bullet leaves the barrel paral ...
and ideal trigger control. However, these features are not well suited for a typical hunting or
action shooting Practical shooting, also known as dynamic shooting or action shooting, is a set of shooting sports where the competitors try to unite the three principles of precision, power, and speed, by using a firearm of a certain minimum power factor to sco ...
rifle, where the rifle must be brought from a ready position to a shooting position quickly and smoothly. This use favors low sights or scopes, and a shallow pistol grip angle. Rounded forearms are well suited for firing from offhand, while a square-bottomed forearm provides a stable base for shooting off of a sandbag or other rest. Recoil is also a key issue in rifle stock design. Heavy recoiling rifles should have wide butts, with a good recoil pad to absorb the force of recoil, and a comb that is straight or slopes down towards the action, so that it does not push into the shooter's face under recoil. Some target shooting disciplines allow the use of various devices to help support the rifle, and these are often mounted to an accessory rail beneath the forend. '' Target slings'', unlike carrying slings, are used just on the off hand, usually with a '' hand stop'', and provide stability to the shooter's hold. '' Palm rests'' are another device that can be attached to the rail, to allow a shooter to lower their off hand and place their elbow into their body for support. Target stocks also are available with a large degree of adjustment, including
length of pull Length of pull (sometimes abbreviated as LOP) is the distance from the trigger to the part of a rifle or shotgun which fits against the shoulder of the shooter. Length of pull is an important ergonomic factor for ease of use; and optimum length of ...
, drop, comb height and angle, and buttplate angle and curvature.


Time considerations

The bullet does not leave the barrel as soon as the sear releases; rather, there is a delay between the release of the sear and the bullet exiting the barrel. During this time, any movement will move the firearm off target, and so this time should be minimized, especially for firearms that will be fired from an unsupported standing position. This delay can be broken down into two sections, the ''lock time'' and the ''bullet dwell time''.


Lock time

Lock time Lock time or action time refers to the time interval (often measured in milliseconds) from when the trigger of a firearm is activated until the firing pin strikes the primer, and depends on the design of the firing mechanism. A long lock time inc ...
is the time between the release of the sear and the ignition of the cartridge, and depends on the design of the firing mechanism. A lengthy lock time gives time for the shooter to drift off target, and so it is advantageous to minimize the lock time and reduce the window for error. Reductions in lock time are generally performed by lightening parts that move as part of the firing operation, such as the
hammer A hammer is a tool, most often a hand tool, consisting of a weighted "head" fixed to a long handle that is swung to deliver an impact to a small area of an object. This can be, for example, to drive nails into wood, to shape metal (as wi ...
and firing pin or striker, shortening the distance that parts, moving as components of the firing operation, have to cover, and using a more powerful spring. A short lock time is particularly desired when shooting with high precision at small targets. The lock time of conventional bolt-action rifles typically lie between 2.6 to 9.0 milliseconds. Further reductions in lock time, to near zero levels, can be achieved with electrical primers.


Bullet dwell time

The bullet dwell time is the time between ignition of the cartridge and when the bullet leaves the barrel. Like lock time, dwell time is a window for error, and can be minimized with a faster bullet or a shorter barrel. In some cases, a shorter barrel is desired to reduce dwell time, but without losing the sight radius of a longer barrel. In this case, a sight extension tube, or ''bloop tube'', can be used. This is a tube that fits on the muzzle end of the barrel, providing support for the front sight, but that is bored to much larger than bore diameter. This provides the sight plane of a long barrel with less weight and dwell time. To put lock time and bullet dwell time in perspective; the lock time of most conventional
bolt action rifle Bolt-action is a type of manual firearm action that is operated by ''directly'' manipulating the bolt via a bolt handle, which is most commonly placed on the right-hand side of the weapon (as most users are right-handed). Most bolt-action ...
s varies between 2.6 and 9.0
millisecond A millisecond (from '' milli-'' and second; symbol: ms) is a unit of time in the International System of Units (SI) equal to one thousandth (0.001 or 10−3 or 1/1000) of a second and to 1000 microseconds. A unit of 10 milliseconds may be calle ...
s, while after cartridge ignition most rifle bullets travel through a high powered rifle barrel bore in 1.0 to 1.5 milliseconds. Mechanical bolt-action rifle trigger systems with a sub 2.0 millisecond lock time are applied in most purpose designed high end competition rifles.


Clearances and Tolerances

The terms "clearance" and "tolerance" are very often confused and misused. Clearance is the distance between the surfaces of mating parts. Tolerance is the allowable variation of a dimension from its nominal (desired) value. For example, a bolt with an outside diameter (OD) of 0.697 inches which is operating in a receiver with a bolt raceway inside diameter (ID) of 0.702 has a clearance of 0.005 inches. If the bolt OD has a nominal OD of 0.698 and a tolerance of +/- 0.001, then the bolt OD can randomly vary from 0.697 to 0.699 along its length, which will allow the bolt clearance in the 0.702 diameter raceway to vary from 0.005 to 0.003. Further, if the raceway also has a tolerance of +/- 0.001 from the nominal of 0.702, then its ID can vary from 0.701 to 0.703 along its length. That combination of tolerances can allow the bolt clearance to vary from 0.002 to 0.006. The point at which the 0.002 clearance occurred would likely cause binding and malfunction in most operational environments. To ensure consistent, repeatable lockup, clearances between
moving parts Machines include both fixed and moving parts. The moving parts have controlled and constrained motions. Moving parts are machine components excluding any moving fluids, such as fuel, coolant or hydraulic fluid. Moving parts also do not include ...
must be kept to the smallest value that will allow proper operation of the mechanism. This goal can be achieved by carefully hand selecting parts and precisely fitting them together, or by manufacturing new parts (bolt, receiver, barrel, etc. to precise dimensions using much tighter tolerances than production components. The best fits are generally attained by selecting slightly oversized parts (or altering stock parts to form an
interference fit An interference fit, also known as a pressed fit or friction fit is a form of fastening between two ''tight'' fitting mating parts that produces a joint which is held together by friction after the parts are pushed together. Depending on the am ...
) and then
lapping Lapping is a machining process in which two surfaces are rubbed together with an abrasive between them, by hand movement or using a machine. Lapping often follows other subtractive processes with more aggressive material removal as a first ste ...
the mating surfaces to attain the desired fit (clearance). Clearances cannot be too tight, however, or functionality will be compromised; this is very important in automatic and
semi-automatic firearm A semi-automatic firearm, also called a self-loading or autoloading firearm ( fully automatic and selective fire firearms are also variations on self-loading firearms), is a repeating firearm whose action mechanism ''automatically'' loads a fol ...
s, where there is a certain amount of energy extracted from the firing of the cartridge that must be used to cycle the action. Overly tight clearances mean that there is no place for lubricant and dirt to be, and this can bind the parts. However, except for cost considerations, it is always beneficial to minimize the tolerances applied to the manufacture of mating parts. In some cases, stock parts are not sufficient for the task of producing a finished product with sufficiently tight clearances. In this case, it may be necessary to use specially fabricated parts, built either with minimal clearances (but to a very tight tolerance), or parts that are oversized and intended for hand fitting.


Barrel

The barrel is one of the most important factors for accuracy, as a poorly made barrel may be beyond correction. Even a quality barrel must be well matched to the cartridge it will be firing. In most cases, it is not practical to fix a barrel whose bore is worn out, poorly or unsuitably rifled, or the wrong bore diameter; the primary exception to that is rimfire barrels, which can be inexpensively bored out and re-lined with a commercial barrel liner. If the barrel is unsuitable and relining is not an option, then an aftermarket or custom barrel is the best solution. However, if the bore is good, then there are a number of operations that can be done to the barrel to improve upon its accuracy.


Bore

Ideally, the bore must be cylindrical and the rifling geometry the same down the length of the bore. Some airgun rifles have a short cone towards the muzzle to improve the speed of the bullet. A slight gain in the rifling twist, or a slight tapering down of the bore, will still ensure the tight fit of bullet to bore, so when selecting a barrel, the tighter or faster twist end should be chosen for the muzzle. The twist of the rifling must match the intended ammunition for best accuracy. Rifling with too slow a twist will not stabilize long bullets, causing them to
precess Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In othe ...
in flight; at its worst, this can result in the bullets tumbling in-flight and ''keyholing'', where bullets strike the target sideways. Too fast a twist can also be a problem, as it can magnify problems in the bullet. A bullet whose center of mass is slightly off-center will diverge at a rate proportional to the rifling twist, so excess twist will result in greater dispersion. In practical terms, this is only a problem for rifles chambered for common military calibers where a variety of different loads exist. For example, the
M16A1 The M16 rifle (officially designated Rifle, Caliber 5.56 mm, M16) is a family of military rifles adapted from the ArmaLite AR-15 rifle for the United States military. The original M16 rifle was a 5.56×45mm automatic rifle with a 20-roun ...
rifle is unable to accurately fire bullets that are heavier than due to barrel twist that is too slow to stabilize heavier bullets. Precision rifles typically come with barrels that are either tailor-made for a specific ammunition load or made according to the buyer's specifications. Barrels can also benefit from
lapping Lapping is a machining process in which two surfaces are rubbed together with an abrasive between them, by hand movement or using a machine. Lapping often follows other subtractive processes with more aggressive material removal as a first ste ...
, not only because it makes the bore a more consistent diameter, but also because it polishes the bore. Barrel lapping should be done with the tool moving in the same direction as the bullet will move, so that any imperfections in the barrel will be smoothed out and thus not interfere with the passage of the bullet. A smooth, polished bore will not only hold the bullet better, but also reduce barrel fouling.


Chamber

Of great importance to accuracy is the fit of the ammunition to the barrel. The chamber should be concentric, and the throat sized just slightly larger than the bullet diameter. The cartridge must headspace correctly, be held firmly in place, concentric to the bore, and the bullet must fit the bore and be guided to engage the
rifling In firearms, rifling is machining helical grooves into the internal (bore) surface of a gun's barrel for the purpose of exerting torque and thus imparting a spin to a projectile around its longitudinal axis during shooting to stabilize the ...
cleanly. Once the bullet is engaged in the rifling, with a good seal and coaxial fit, it must stay that way. It is often possible to shorten a barrel slightly by removing material from the breech end and re-cut the chamber, which can correct many problems in the original chamber.


Crown

The crown is the muzzle face of the barrel. The crown's integrity is critical for two reasons: #It is the last part of the firearm touched by the bullet before it exits. #As the bullet clears the crown, it will release upwards of of back pressure, which must be as uniform as possible. While many barrel manufacturers recess the crown to protect it from accidental damage, it can still be damaged over time by cleaning rods that are too hard. It is also not uncommon for factory crowns to be cut slightly off-center, so that one side of the bullet exits slightly earlier than the other, and this will result in the bullet being pushed away from that side, causing a significant deflection in its path. A crown may be re-cut with relative ease, and this can fix any problems caused by a defective or damaged crown, by ensuring an even release of the bullet.


Stress

Any machining process on a barrel, whether it's boring, rifling, or turning the outside contour, will produce some stress in the steel of the barrel. This stress can cause the barrel to expand unevenly as it heats, causing shots to "walk" as the barrel heats and cools. To prevent this, careful post-machining heat treatment is often used to stress-relieve the barrels. The amount of good this does depends on the technique used to make the barrel. For example, the hammer forging method of manufacture leaves a significant amount of stress in the barrels, which could be addressed through stress-relieving heat treatment.


Wear

Barrel wear is also an important issue, particularly in high-powered firearms. High temperatures tend to erode the barrel at the throat, preventing the bullet from entering the rifling cleanly. One way to produce a long-lasting barrel is by the proper selection of materials.
Stainless steel Stainless steel is an alloy of iron that is resistant to rusting and corrosion. It contains at least 11% chromium and may contain elements such as carbon, other nonmetals and metals to obtain other desired properties. Stainless steel's r ...
s, such as 416, have been shown to have a longer life than the traditional 4140 chrome/
molybdenum Molybdenum is a chemical element with the symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with lead ...
steels used for barrels. While the stainless barrels are not any more accurate than a 4140 barrel, they will maintain their accuracy longer in many applications, as they are more resistant to erosion caused by the heat of firing high-powered cartridges. One notable exception to this is the .50 BMG cartridge; competition shooters often fire high-precision
lathe A lathe () is a machine tool that rotates a workpiece about an axis of rotation to perform various operations such as cutting, sanding, knurling, drilling, deformation, facing, and turning, with tools that are applied to the workpiece ...
-turned bullets made from harder brass, bronze, or steel in this caliber, and the 4140 steel will stand up to this better than stainless.


Cryogenic treatment

Another commonly cited accurizing treatment for barrels is
cryogenic treatment A cryogenic treatment is the process of treating workpieces to cryogenic temperatures (i.e. below ) in order to remove residual stresses and improve wear resistance in steels and other metal alloys, such as aluminum. In addition to seeking enhanc ...
. This involves slowly cooling the steel down to
liquid nitrogen Liquid nitrogen—LN2—is nitrogen in a liquid state at low temperature. Liquid nitrogen has a boiling point of about . It is produced industrially by fractional distillation of liquid air. It is a colorless, low viscosity liquid that is wid ...
temperatures, leaving it there for a time, and then slowly warming back to room temperature. This process converts remaining
austenite Austenite, also known as gamma-phase iron (γ-Fe), is a metallic, non-magnetic allotrope of iron or a solid solution of iron with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of 100 ...
in the steel to
martensite Martensite is a very hard form of steel crystalline structure. It is named after German metallurgist Adolf Martens. By analogy the term can also refer to any crystal structure that is formed by diffusionless transformation. Properties M ...
. Many proponents of this process claim increased accuracy of the resulting barrels, but independent testing of the process by major manufacturers has shown no increase in accuracy. However, the conversion of austenite to martensite has been shown to result in easier machining and greater wear resistance on steels that tend to have significant amounts of retained austenite, such as stainless steels, and the process does appear to significantly affect the accurate life of stainless steel barrels.


Rifle stocks

A poor fit of the action to the stock is also a source of problems, and this problem is exacerbated by issues such as
thermal expansion Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions. Temperature is a monotonic function of the average molecular kin ...
of metal parts during use, and swelling and contracting of wooden stocks with changes in
humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity dep ...
. These changes can affect accuracy either by allowing the action to shift under recoil, or by causing slight but accuracy-destroying bending of the barrel. By removing wood from areas of contact and, when needed, replacing it with a more stable substance that is precisely fitted, such as a fiberglass composite molded in place, a more stable, precise fit can be achieved. Other materials, such as composites or laminated wood, can also provide a stronger, more dimensionally stable stock than traditional woods can. Some stocks are even being made out of aluminum or other metals, for maximum stability. The process of fitting the action to the stock is called ''
bedding Bedding, also known as bedclothes or bed linen, is the materials laid above the mattress of a bed for hygiene, warmth, protection of the mattress, and decorative effect. Bedding is the removable and washable portion of a human sleeping environm ...
'', and there are a number of different processes used. ''Glass bedding'' uses a fiberglass composite which is molded in place around the action. This can be as simple as bedding the recoil lug of a
bolt-action Bolt-action is a type of manual firearm action that is operated by ''directly'' manipulating the bolt via a bolt handle, which is most commonly placed on the right-hand side of the weapon (as most users are right-handed). Most bolt-actio ...
rifle, or as complex as bedding the entire action and barrel channel. ''Pillar bedding'' uses precisely machined metal pillars in the stock which mate with the action, providing a strong metal-to-metal contact without the labor-intensive hand fitting required in glass bedding. Many times it is advantageous to eliminate most or all contact between the stock and barrel, to eliminate potential accuracy-destroying interference in the barrel harmonics. To do this, stock material is removed along the barrel channel to leave a slight gap between the stock and the barrel; this is called ''free floating'' the barrel. With some designs, ''pressure bedding'' is preferred, where a single point of contact remains between stock and barrel near the forend. In both these cases, bedding the action is required to provide support for the floating or pressure bedded barrel. Since the action ends up supporting the mass of the barrel, a poor stock to action fit will result in unacceptable shifting.


Action

The primary purposes of the
firearm action In firearms terminology, an action is the functional mechanism of a breech-loading firearm that handles (loads, locks, fires, extracts and ejects) the ammunition cartridges, or the method by which that mechanism works. Actions are technically n ...
are holding the cartridge in place in the chamber, and providing a way to ignite the propellant. In a
single-shot Single-shot firearms are firearms that hold only a single round of ammunition, and must be reloaded manually after every shot. The history of firearms began with single-shot designs, then multi-barreled designs appeared, and eventually many cent ...
action, little additional functionality is provided, while in a
semi-automatic firearm A semi-automatic firearm, also called a self-loading or autoloading firearm ( fully automatic and selective fire firearms are also variations on self-loading firearms), is a repeating firearm whose action mechanism ''automatically'' loads a fol ...
the action also taps energy from the firing process for cycling to fire the next round. From an accuracy perspective, the primary goal of the action is to achieve a consistent placement of the cartridge in the chamber every shot. The term blueprinting, borrowed from high performance engine builders, is also often applied to the process of manufacturing or altering parts to have a desired (usually tighter) clearance than stock parts, and tightening the tolerances on critical dimensions to reduce clearance variations. Blueprinting a firearm action involves similar operations designed to tighten the clearances of a firearm action to ensure consistent and correct seating of the cartridge in the chamber. A typical set of blueprinting operations for a bolt-action firearm would include the following: #Bolt blueprinting operations: ##Squaring the bolt face to the bolt diameter ##Ensuring the bolt is concentric in the action ##Squaring and lapping the locking lugs ##Sleeving the bolt (adding material to increase the diameter), then machining down for a precise fit to the action #Receiver blueprinting operations: ##Making the
action threads In firearms, barrel threads refer to the screw threads used to attach a barrel. Action threads, also called receiver threads, are situated at the chamber end of the barrel, and can be used for attaching the barrel to the receiver. The receiver ...
for the barrel concentric with the action centerline ##Making sure the breech face is square to the action centerline ##Making sure the recoil lug is square to the action ##Squaring and lapping the locking lug recesses These operations ensure that not only is the cartridge consistently and correctly positioned when chambered, but also that it remains correctly positioned during firing.


Revolver specific issues

The defining characteristic of a
revolver A revolver (also called a wheel gun) is a repeating firearm, repeating handgun that has at least one gun barrel, barrel and uses a revolving cylinder (firearms), cylinder containing multiple chamber (firearms), chambers (each holding a single ...
is the revolving cylinder, separate from the barrel, that contains the chambers. Revolvers typically have 5 to 9 chambers, and the first issue is ensuring consistency among the chambers; if they are not consistent then the point of impact will vary from chamber to chamber. The chambers must also align consistently with the barrel, so the bullet enters the barrel the same way from each chamber. The throat in a revolver is part of the cylinder, and like any other chamber, the throat should be sized so that it is concentric to the chamber and very slightly over the bullet diameter. At the end of the throat, however, things change. First, the throat in a revolver is at least as long as the maximum overall length of the cartridge; if otherwise the cylinder cannot revolve. The next step is the cylinder gap, the space between the cylinder and barrel. This must be wide enough to allow free rotation of the cylinder even when it becomes fouled with powder residue, but not so large that excess gas can be released. The next step is the forcing cone. The forcing cone is where the bullet is guided from the cylinder into the bore of the barrel. It should be concentric with the bore, and deep enough to force the bullet into the bore without significant deformation. Unlike rifles, where the threaded portion of the barrel is in the chamber, revolver barrels threads surround the breech end of the bore, and it is possible that the bore will be compressed when the barrel is screwed into the frame. Cutting a longer forcing cone can relieve this "choke" point, as can lapping of the barrel after it is fitted to the frame. A consistent lockup is important to keep all these parts in line, and revolvers are prone to abuse that can damage these parts, adversely affecting the accuracy and even safety of the revolver. This lockup consists of two parts, the crane to frame lockup, and the cylinder bolt to cylinder lockup. Many swing-out cylinder revolvers only support the cylinder securely at the rear, and flipping the cylinder open and closed can bend the crane and prevent the cylinder from lining up parallel to the bore. The cylinder bolt, which engages the bottom of the cylinder through a slot in the frame, should provide a relatively tight lockup, and not drag the cylinder during rotation or pop loose when the hammer is cocked at a reasonable speed. Fanning a revolver can batter the cylinder bolt and prevent a solid lockup.


Harmonics

During firing, the chamber pressure rises from atmospheric pressure to, in a typical rifle cartridge, pressures of about within microseconds. This rapid increase in pressure causes the barrel to vibrate at a certain natural
frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
, much like a
tuning fork A tuning fork is an acoustic resonator in the form of a two-pronged fork with the prongs ( tines) formed from a U-shaped bar of elastic metal (usually steel). It resonates at a specific constant pitch when set vibrating by striking it agains ...
. The point in time at which the bullet exits the barrel will determine the orientation of the muzzle relative to its rest position. Exiting near a peak or valley in the motion means the muzzle is relatively stationary, and shot dispersion will be minimized; exiting between a peak and valley means the muzzle is rapidly moving, and shot dispersion will be greater. There are two ways to address harmonics; reducing the amplitude with a stiffer barrel, or working with the natural frequency to minimize dispersion.


Stiffness

Stiffness of a barrel is proportional to the fourth power of the diameter, and inversely proportional to the third power of the length. Because of this, short, thick barrels will vibrate with high frequency and low amplitude, and long, thin barrels will vibrate with a low frequency and high amplitude. Due to the effect of length, barrel harmonics are primarily a concern with rifles. By using the shortest and/or fattest barrel possible, the amplitude of the vibrations can be minimized to the point that they are irrelevant to accuracy. Unlimited class
benchrest shooting Benchrest shooting is a shooting sport discipline in which high-precision rifles are rested on a table or bench — rather than being carried in the shooter's hands — while shooting at paper or steel targets, hence the name "benchrest". ...
barrels, where weight is of very little consequence, have very large diameters; an outside diameter of 2 inches (5 cm) is not uncommon. While standard rifle barrels taper from breech to muzzle, high precision rifles will often use a barrel with far less taper, called a ''heavy barrel'', sometimes leaving the barrel cylindrical all the way to the muzzle, called a ''bull barrel''. Either technique greatly increases the stiffness of the barrel by enlarging the average diameter, but this process adds significant weight as well. This can greatly increase the mass of the barrel, however; going from a lightweight sporter contour to a heavy barrel contour can double the mass, and a going to a bull barrel contour can more than triple it. Fluting, consisting of grooves machined in the outer surface of the barrel to remove material, can reduce the weight and improve heat dispersion while maintaining most of the stiffness. Barrel tensioning devices are a way of gaining stiffness with minimal increases in weight. They do this by placing a lightweight sleeve, often made of aluminium or a
carbon fiber Carbon fiber-reinforced polymers (American English), carbon-fibre-reinforced polymers (Commonwealth English), carbon-fiber-reinforced plastics, carbon-fiber reinforced-thermoplastic (CFRP, CRP, CFRTP), also known as carbon fiber, carbon compo ...
composite, around the barrel, and then using a nut attached to the end of the barrel to tension the barrel and place the sleeve under compression. This serves to keep the muzzle closer to concentric and coaxial to the breech during vibration.


Harmonic tuning

The other solution is to work with the barrel's natural vibration, and tune the components so that the bullet exits the barrel as it is moving the slowest. The simplest approach to harmonic tuning is to concentrate on the ammunition. The
internal ballistics Internal ballistics (also interior ballistics), a subfield of ballistics, is the study of the propulsion of a projectile. In guns, internal ballistics covers the time from the propellant's ignition until the projectile exits the gun barrel. The s ...
of a given cartridge will determine its ''dwell time'', or the time it takes from ignition to exiting the barrel. By experimentally matching the dwell time to the barrel's frequency, the best load for a particular firearm may be found. Similarly,
handloading Handloading, or reloading, is the practice of making firearm cartridges by assembling the individual components (case, primer, propellant, and projectile), rather than purchasing mass-assembled, factory-loaded ammunition. The term ''handlo ...
gives the shooter the opportunity to very precisely control the bullet velocity, and experimentally choose the optimum velocity. If it is not possible or desirable to match the bullet to the barrel, there are a number of devices marketed to allow the barrel to be tuned to match the ammunition. There are a number of models of these that work in different ways. One type uses an adjustable damper or pressure bedding point to allow the shooter to find the "sweet spot", where it will do the most good at damping the vibrations that are affecting accuracy. Other tuners work by using an adjustable weight on the muzzle to alter the length of the resonant portion of the barrel, and allowing the frequency to be matched to the ammunition.


Airgun powerplants

The difference between an airgun and a firearm is the way in which the power to launch the projectile is provided. In a firearm the projectile propulsion is provided by an exothermic chemical reaction, and in an airgun is it provided primarily by mechanically compressed gas, typically either air or
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
(CO2), though these gases are used primarily for convenience and some airgun variants run on other gases, such as
refrigerant A refrigerant is a working fluid used in the refrigeration cycle of air conditioning systems and heat pumps where in most cases they undergo a repeated phase transition from a liquid to a gas and back again. Refrigerants are heavily regulated ...
s like
R-134a 1,1,1,2-Tetrafluoroethane (also known as norflurane (INN), R-134a, Freon 134a, Forane 134a, Genetron 134a, Green Gas, Florasol 134a, Suva 134a, or HFC-134a) is a hydrofluorocarbon (HFC) and haloalkane refrigerant with thermodynamic properties ...
commonly used in
airsoft Airsoft is a team game in which participants eliminate opposing players by tagging them out of play with spherical plastic projectiles shot with mock air weapons(usually powered by an electronic motor) called airsoft guns. Although similar ...
guns, or
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
used in
light-gas gun The light-gas gun is an apparatus for physics experiments. It is a highly specialized gun designed to generate extremely high velocities. It is usually used to study high-speed impact phenomena (hypervelocity research), such as the formation of ...
s. There are three primary types of powerplant used in airguns: *Spring-piston, which uses a spring-loaded piston to compress air within an
air pump An air pump is a pump for pushing air. Examples include a bicycle pump, pumps that are used to aerate an aquarium or a pond via an airstone; a gas compressor used to power a pneumatic tool, air horn or pipe organ; a bellows used to encourage a ...
at the moment of firing *Pneumatic, which uses pre-compressed air stored in a reservoir within the gun *Compressed gas, which uses a small removable
gas cylinder A gas cylinder is a pressure vessel for storage and containment of gases at above atmospheric pressure. High-pressure gas cylinders are also called ''bottles''. Inside the cylinder the stored contents may be in a state of compressed gas, vap ...
now ubiquitously stores liquid CO2 (
Powerlet Crosman Corporation is an American designer, manufacturer and supplier of shooting sport products, with a long-standing presence in airgun design and a tradition of producing pellet and BB guns. Crosman is also a producer of many varieties of a ...
) Each method has its own advantages and disadvantages, and different areas that can be addressed to ensure consistency. The most powerful systems will produce velocities near or exceeding the
speed of sound The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elastic medium. At , the speed of sound in air is about , or one kilometre in or one mile in . It depends strongly on temperature as we ...
with lightweight pellets; this, however, is not a good thing where accuracy is concerned. The commonly used airgun diabolo pellets have a poor
ballistic coefficient In ballistics, the ballistic coefficient (BC, ''C'') of a body is a measure of its ability to overcome air resistance in flight. It is inversely proportional to the negative acceleration: a high number indicates a low negative acceleration—the ...
, and quickly lose velocity; when they drop below the speed of sound, they will often tumble. However, high velocities sell airguns; if accuracy is desired from these high velocity guns, then heavier pellets should be used to keep the velocity down. This will provide not only better accuracy, but better downrange preservation of velocity and kinetic energy.


Pneumatic

Pneumatic systems use compressed gas for power, usually compressed air. This air may be compressed by the gun for each shot, in a ''single stroke'' or ''pump'' (multiple stroke) gun, or it may be ''precharged'' by an external compressor. A single stroke system, as the name implies, uses a single stroke of the pump to compress a cylinder full of air, which is then all used for one shot of the gun. Single stroke systems are both inexpensive and capable of high accuracy due to the simplicity and consistency of the single stroke design. The downsides are the low power provided, though this is not a handicap in standard 10 meter airgun target shooting. When
Daisy Daisy, Daisies or DAISY may refer to: Plants * ''Bellis perennis'', the common daisy, lawn daisy or English daisy, a European species Other plants known as daisy * Asteraceae, daisy family ** '' Euryops chrysanthemoides'', African bush daisy ** ' ...
introduced an inexpensive single stroke pistol, the model 717, in the late 1970s, American
Olympic Olympic or Olympics may refer to Sports Competitions * Olympic Games, international multi-sport event held since 1896 ** Summer Olympic Games ** Winter Olympic Games * Ancient Olympic Games, ancient multi-sport event held in Olympia, Greece bet ...
shooter Don Nygord demonstrated the potential of the design by shooting an accurized version in a California State air pistol championship, and winning the gold medal. The (at the time) US$40 Daisy, with a better rear sight and an adjustable trigger added, shot as well as the US$400 Olympic class air pistols it was competing against. More powerful is the pump system, which is a slightly more complex version of the single stroke design. Rather than leaving the air in the piston when compressed, the pump airgun has a reservoir to contain the compressed air, allowing multiple pumps to be used, typically 2 at a minimum, up to 10 pumps for full power. The ability to vary the power, however, is the pump airgun's major disadvantage when it comes to accuracy, as it makes it very difficult to get a consistent charge. The increased power of a pump airgun makes it an attractive choice for many shooters, and there are steps that can be taken to improve consistency, such as modifying the air chamber so that not all the air is exhausted in a shot. The last type of pneumatic airgun is the precharged pneumatic. This is both an old and a new design; some of the earliest airguns, such as the model carried by
Lewis and Clark Lewis may refer to: Names * Lewis (given name), including a list of people with the given name * Lewis (surname), including a list of people with the surname Music * Lewis (musician), Canadian singer * "Lewis (Mistreated)", a song by Radiohead ...
, were of this type, as are many new cutting edge models. The precharged pneumatic uses an external source of compressed air, either an external pump or a high pressure reservoir such as a
SCUBA tank A diving cylinder or diving gas cylinder is a gas cylinder used to store and transport high pressure gas used in diving operations. This may be breathing gas used with a scuba set, in which case the cylinder may also be referred to as a scub ...
, to fill a reservoir. The reservoir can be a small, single-shot one, such as in the Brocock Air Cartridge system, or a large, multi-shot tank. The key to top accuracy in a precharged pneumatic is a consistent pressure. With multishot systems (as are most), the pressure in the reservoir will drop with each shot fired, so the best way to achieve consistency is with a
pressure regulator A pressure regulator is a valve that controls the pressure of a fluid or gas to a desired value, using negative feedback from the controlled pressure. Regulators are used for gases and liquids, and can be an integral device with a pressure setti ...
, which provides a steady, but lower, pressure at the valve, for as long as the reservoir pressure remains higher than the regulated pressure. Regulators are also generally adjustable, so a low pressure setting will provide many shots of lower power, while a high pressure setting will provide a few high power shots.


Piston

Piston airguns, often called "springers", are unique in many ways. Since the firing process involves a fairly massive piston suddenly moving to compress the air, they have a significant "kick", generally called "recoil" (though this is not the same as firearm recoil). The recoil begins when the piston starts to move forwards, which pushes the rest of the gun backwards. The recoil then stops suddenly as the piston reaches the end of its travel, and is brought to a stop by the cushion of high-pressure air trapped between the piston and pellet. This recoil can be brutal on the gun in high-powered models, and will loosen screws, shift sights, and break scopes not designed specifically for the unique recoil of piston airgun—all of these can lead to poor accuracy. In addition to the recoil, piston airguns have a long lock time, as the piston must compress the air before the pellet begins to move, and the gun is moving due to the recoil during this time. Spring airguns require a special technique to fire, to ensure that the gun moves very consistently during this recoil. The preferred method is a very loose hold, to allow the gun to move back; this means that a piston airgun will ''not'' shoot the same from a bench. All accuracy testing and sighting in must be done in the same position the gun will be shot from, otherwise the results will be different. Piston airgun recoil cannot readily be dealt with without a significant redesign; in some cases the action can be mounted on a sliding rail, or two counter-recoiling pistons can be used, but this requires significant changes to the design. The resulting gun will be far less sensitive to the shooter's hold, and thus much easier to shoot accurately. The RSW-54 is a model 48/52 with the action mounted on rails, so it can recoil within the stock. The first step to accuracy of a piston gun is to ensure that all screws are secure and the sights are rated for use on a piston airgun. Another potential issue regarding accuracy is resonance in the spring used to power the piston in most airguns. The spring will vibrate strongly as the piston stops, and this will affect the harmonics of the gun. A
gas spring A gas spring is a type of spring that, unlike a typical mechanical spring that relies on elastic deformation, uses compressed gas contained within an enclosed cylinder sealed by a sliding piston to pneumatically store potential energy and wit ...
will, if one can be fitted to a given model, provide vibrationless action, though with some loss of efficiency and even sharper recoil. Spring driven pistons also respond well to accurizing; careful fitting of parts and use of quality lubricants and spring damping ''tar'' can reduce the level of vibrations and improve accuracy Contains links to all sections of article on the tuning of an R1 air rifle


CO2

CO2 is commonly found in multishot airguns, from the cheapest plinkers to Olympic class target guns, though the latter are facing competition from regulated precharged pneumatics. CO2's advantage is that it is stored in a liquid form, rather than a gas, and as such provides a greater power density. The liquid also provides a constant pressure, the
vapor pressure Vapor pressure (or vapour pressure in English-speaking countries other than the US; see spelling differences) or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed pha ...
, as long as there is liquid remaining in the reservoir. The downside to CO2 is that it is dependent upon the vapor pressure, which changes significantly with temperature. This is of primary concern to outdoor shooters, who may shoot in widely varying temperatures, or for rapid fire shooters, as rapid release of the gas results in a rapid drop in the temperature of the liquid. The issue of temperature change is not readily addressable, other than by the use of readily adjustable sights, so the shooter can adjust the sights to match the point of impact based on the current ambient conditions. In the case of rapid fire, there is a solution that can provide far greater stability for many guns. The original CO2 airguns were filled from an external source of CO2, but in 1954 Crosman introduced the 12 gram
Powerlet Crosman Corporation is an American designer, manufacturer and supplier of shooting sport products, with a long-standing presence in airgun design and a tradition of producing pellet and BB guns. Crosman is also a producer of many varieties of a ...
, a compact, disposable tank now ubiquitous in inexpensive CO2 airguns. The downside to these is that the small quantity of CO2 liquid cools down quickly, leading to a rapid drop in velocity and a changing point of impact. By moving to a bulk fill system, with a much larger tank, there is more liquid available, and the greater mass will cool far more slowly.


References

{{Firearms Firearm terminology