The bore axis of a

firearm A firearm is any type of gun that uses an explosive charge and is designed to be readily carried and operated by an individual. The term is legally defined further in different countries (see legal definitions). The first firearms originate ...

is the longitudinal axis through the geometric center of the

gun barrel A gun barrel is a crucial part of gun-type weapons such as small arms, small firearms, artillery pieces, and air guns. It is the straight shooting tube, usually made of rigid high-strength metal, through which a contained rapid expansion of high ...

. In a rifled barrel, 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 ...

(

bullet A bullet is a kinetic projectile, a component of firearm ammunition that is shot from a gun barrel. They are made of a variety of materials, such as copper, lead, steel, polymer, rubber and even wax; and are made in various shapes and constru ...

ball A ball is a round object (usually spherical, but sometimes ovoid) with several uses. It is used in ball games, where the play of the game follows the state of the ball as it is hit, kicked or thrown by players. Balls can also be used for s ...

, pellet or

slug Slug, or land slug, is a common name for any apparently shell-less Terrestrial mollusc, terrestrial gastropod mollusc. The word ''slug'' is also often used as part of the common name of any gastropod mollusc that has no shell, a very reduced ...

) will spin around the bore axis as it goes through the barrel. Boresighting is a process of placing one's line of sight down along the bore axis.

Bore-to-sight distance

The distance between the bore axis and the sight axis (the

optical axis An optical axis is an imaginary line that passes through the geometrical center of an optical system such as a camera lens, microscope or telescopic sight. Lens elements often have rotational symmetry about the axis. The optical axis defines ...

of a sighting device), also known as the ''sight height'', is an important factor to consider due to

parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different sightline, lines of sight and is measured by the angle or half-angle of inclination between those two lines. Due to perspective (graphica ...

principles. It is normally desirable to keep the sights of a firearm as close to the bore as possible, since a firearm with a large bore-to-sight distance will require more compensatory sight adjustment when shifting between shooting at different targets at close ranges (due to

foreshortening Linear or point-projection perspective () is one of two types of 3D projection, graphical projection perspective in the graphic arts; the other is parallel projection. Linear perspective is an approximate representation, generally on a fla ...

). On the other hand, a firearm with a short bore-to-sight distance will need less sight adjustment when changing between targets at close ranges. At longer ranges the bore-to-sight distance will be of less importance, since

gravity In physics, gravity (), also known as gravitation or a gravitational interaction, is a fundamental interaction, a mutual attraction between all massive particles. On Earth, gravity takes a slightly different meaning: the observed force b ...

has affected the projectile so much that the height difference between the bore axis and the sight axis has far less impact compared to the bullet drop. Traditional bolt hunting rifles usually have had the optical center of about above the bore, while AR-type rifles and modern bolt-action rifles usually have their optical center about above the bore. On firearms with a picatinny rail, the distance from the bore to the optical axis can be divided into distance from bore axis to top of rail, and distance from top of rail to center of the optical axis. For example on an AR-type rifle, the mil-spec distance from center of bore to top of rail is 1.211 inches, or approximately . The mount height itself, that is the distance from the top of rail to center of scope, varies, but typically lies around (higher and lower mounts are also readily available). In total, this gives a typical scoped AR-style rifle a height over bore of around .

Recoil control

The

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, for according to Newton's third law the force requ ...

from a fired cartridge (and the action movement) exerts a rearward impulse along the bore axis, which is commonly above the

center of mass In physics, the center of mass of a distribution of mass in space (sometimes referred to as the barycenter or balance point) is the unique point at any given time where the weight function, weighted relative position (vector), position of the d ...

of the gun. Meanwhile, the gripped areas of the gun, where the user will exert a reactional forward push countering the recoil, is almost always below the gun's center of mass. This force couple creates a

rotation Rotation or rotational/rotary motion is the circular movement of an object around a central line, known as an ''axis of rotation''. A plane figure can rotate in either a clockwise or counterclockwise sense around a perpendicular axis intersect ...

torque In physics and mechanics, torque is the rotational analogue of linear force. It is also referred to as the moment of force (also abbreviated to moment). The symbol for torque is typically \boldsymbol\tau, the lowercase Greek letter ''tau''. Wh ...

around the mass center and make the gun pitch upwards after each shot — a phenomenon known as ''

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" ...

''. This is more prominent in shorter-barreled weapons such as handguns due to less frontal weight available to counter the upward lifting of the muzzle, and how high the bore axis sits above the gripping hand will also affect the degree of muzzle rise. A lower bore axis will align more of the recoil directly into the hand, creating less couple torque and thus less muzzle rise. Among other things that affect muzzle rise and felt recoil is the weight of the pistol frame and tuning of recoil springs. The presence of a buttstock in a

long gun A long gun is a category of firearms with long Gun barrel, 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 w ...

will also help negate the effect of muzzle rise by shifting the pivot of the muzzle rise back towards the butt end, thus utilizing the entire gun's weight with a longer

lever A lever is a simple machine consisting of a beam (structure), beam or rigid rod pivoted at a fixed hinge, or '':wikt:fulcrum, fulcrum''. A lever is a rigid body capable of rotating on a point on itself. On the basis of the locations of fulcrum, l ...

to counter the rise.

Bore axis in pistols

Handguns lack a buttstock to aid in recoil control, so design elements that reduce felt recoil or muzzle flip are considered highly desirable and numerous manufacturers claim their handgun design has a low bore axis, although few offer measurements to prove this claim. While a lower bore axis does typically yield reduced felt recoil and muzzle flip, shooters intent on reducing these features as much as possible (such as competitive sport shooters) will typically seek out handguns with heavy steel frames, as increasing the weight of the gun's non-reciprocating mass (the frame) yields greater reductions in felt recoil and muzzle flip than any difference in bore axis height can typically achieve.{{Cite web, url=https://www.guntweaks.com/beyond-bore-axis-decreasing-recoil-in-pistols.html, title=Handgun Recoil Characteristics and Lowering Felt Recoil, website=GunTweaks.com Handguns such as the CZ-75, which combine a heavy steel frame with a low bore axis (relative to other hammer-fired guns) achieve twofold reductions to felt recoil and muzzle flip, and are popular choices in many competitive shooting disciplines. Striker-fired handgun designs are typically able to achieve a lower bore axis than their hammer-fired counterparts since strikers travel straight forward, rather than up and forward like a typical hammer mechanism. One pistol which combines a low bore axis, striker fire and weight is the Wildebeest pistol.

References

External links

GunTweaks.com - Pistol Bore Axis: Comparison and Explanation
Firearm terminology

Bore-to-sight distance

Recoil control

Bore axis in pistols

See also

References

External links