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Blowback is a system of operation for
self-loading 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 follow ...
firearms that obtains energy from the motion of the
cartridge case A cartridge or a round is a type of pre-assembled firearm ammunition packaging a projectile (bullet, shot, or slug), a propellant substance (usually either smokeless powder or black powder) and an ignition device (primer) within a metallic, ...
as it is pushed to the rear by expanding gas created by the ignition of the
propellant charge A propellant (or propellent) is a mass that is expelled or expanded in such a way as to create a thrust or other motive force in accordance with Newton's third law of motion, and "propel" a vehicle, projectile, or fluid payload. In vehicles, the e ...
. Several blowback systems exist within this broad principle of operation, each distinguished by the methods used to control bolt movement. In most actions that use blowback operation, the
breech Breech may refer to: * Breech (firearms), the opening at the rear of a gun barrel where the cartridge is inserted in a breech-loading weapon * breech, the lower part of a pulley block * breech, the penetration of a boiler where exhaust gases leave ...
is not locked mechanically at the time of firing: the inertia of the bolt and recoil , relative to the weight of the bullet, delay opening of the breech until the bullet has left the barrel. A few locked breech designs use a form of blowback (example: primer actuation) to perform the unlocking function. The blowback principle may be considered a simplified form of
gas operation Gas-operation is a system of operation used to provide energy to operate locked breech, autoloading firearms. In gas-operation, a portion of high-pressure gas from the cartridge being fired is used to power a mechanism to dispose of the spent ...
, since the cartridge case behaves like a piston driven by the powder gases. Other operating principles for self-loading firearms include delayed blowback,
blow forward Blow-forward is a firearm action where the propellant gas pressure and the friction of the bullet traveling down the bore drag the whole gun barrel forward to facilitate the opening of the breech. This forward barrel motion provides most of t ...
,
gas operation Gas-operation is a system of operation used to provide energy to operate locked breech, autoloading firearms. In gas-operation, a portion of high-pressure gas from the cartridge being fired is used to power a mechanism to dispose of the spent ...
, and recoil operation.


Principle of operation

In firearms, a blowback system is generally defined as an operating system in which energy to operate the firearm's various mechanisms, and automate the loading of another cartridge, is derived from the inertia of the spent cartridge case being pushed out the rear of the chamber by rapidly expanding gases produced by a burning propellant, typically gunpowder. When a
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 in ...
(e.g.
bullet A bullet is a kinetic projectile, a component of firearm ammunition that is shot from a gun barrel. Bullets are made of a variety of materials, such as copper, lead, steel, polymer, rubber and even wax. Bullets are made in various shapes and co ...
) is still within the gun barrel, the high-pressure propellant gas behind it is contained within what could be seen as a
closed system A closed system is a natural physical system that does not allow transfer of matter in or out of the system, although — in contexts such as physics, chemistry or engineering — the transfer of energy (''e.g.'' as work or heat) is allowed. In ...
; but at the moment it exits the muzzle, this functional
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 impr ...
is broken, allowing the propellant gas to be suddenly released in an explosive
muzzle blast A muzzle blast is an explosive shockwave created at the muzzle of a firearm during shooting. Before a projectile leaves the gun barrel, it obturates the bore and "plugs up" the pressurized gaseous products of the propellant combustion behind i ...
. The expanding gas also creates a jet propulsion effect rearward in the barrel against the spent cartridge case. This "blowback" is the predominant component of the recoil. Some guns use energy from blowback to perform the automatic bolt cycling /reloading process, while others will use a portion of the blowback to operate only certain parts of the cycle or simply use the blowback energy to enhance the operational energy from another system of automatic operation. What is common to all blowback systems is that the cartridge case must move under the direct action of the powder pressure, therefore any gun in which the bolt is not rigidly locked, and permitted to move while there remains gas pressure in the chamber, will undergo a degree of blowback action. The energy from the expansion of gases upon firing appears in the form of kinetic energy transmitted to the bolt mechanism, which is controlled and used to operate the firearm's operation cycle. The extent to which blowback is employed largely depends on the manner used to control the movement of the bolt and the proportion of energy drawn from other systems of operation. How the movement of the bolt is controlled is where blowback systems differ. Blowback operation is most often divided into three categories, all using residual pressure to complete the cycle of operation: simple blowback (often just "blowback"), delayed/retarded blowback, and advanced primer ignition. Relating blowback to other types of automatic firearm operation, George M. Chinn wrote that: "In the larger sense, blowback might well be considered a special form of gas operation. This is reasonable because the cartridge case may be conceived of as a sort of piston driven by the powder gases. Actually, blowback involves so many special problems that it is best considered to be in a class by itself. The question whether or not it should be included within the more general class of
gas operation Gas-operation is a system of operation used to provide energy to operate locked breech, autoloading firearms. In gas-operation, a portion of high-pressure gas from the cartridge being fired is used to power a mechanism to dispose of the spent ...
or recoil operation is purely academic. The important point is that it partakes some of the properties of both classes and, depending on the particular problem at hand, may be considered to be either one."


Simple blowback

The blowback (sometimes referred to as "simple", "straight" or "pure" blowback) system represents the most basic auto loading operation type. In a blowback mechanism, the bolt rests against the rear of the barrel, but is not locked in place. At the point of ignition, expanding gases push the bullet forward through the barrel while at the same time pushing the case rearward against the bolt. The expanding gases push the bolt assembly to the rear, but the motion is slowed by the mass of the bolt, internal friction, and the force required to compress the action spring. The design must ensure that the delay is long enough that the bullet exits the barrel before the cartridge case clears the chamber. The empty case is ejected as the bolt travels to the rear. The stored energy of the compressed action spring then drives the bolt forward (although not until the trigger is pulled if the weapon fires from an open bolt). A new cartridge is stripped from the magazine and chambered as the bolt returns to its in-battery position. The blowback system is practical for firearms using relatively low-power cartridges with lighter weight bullets. Higher power cartridges require heavier bolts to keep the breech from opening prematurely; at some point, the bolt becomes too heavy to be practical. For an extreme example, a 20 mm cannon using simple blowback and lubricated cartridges would need a bolt to keep the cartridge safely in the barrel during the first few milliseconds; furthermore, the average force supplied by the return spring is limited to or the bolt will not travel back far enough to feed a new round. Consequently, the return spring is not powerful enough to keep the bolt closed when the gun is tilted up. In addition, there is not enough energy stored in the bolt to cycle the weapon. Due to the required bolt weight, blowback designs in pistols are generally limited to calibers smaller than
9×19mm Parabellum The 9×19mm Parabellum (also known as 9mm Parabellum or 9mm Luger or simply 9mm) is a rimless, tapered firearms cartridge. Originally designed by Austrian firearm designer Georg Luger in 1901, it is widely considered the most popular handgu ...
(e.g.,
.25 ACP The .25 ACP ( Automatic Colt Pistol) (6.35×16mmSR) is a semi-rimmed, straight-walled centerfire pistol cartridge introduced by John Browning John Moses Browning (January 23, 1855 – November 26, 1926) was an American firearm designe ...
,
.32 ACP .32 ACP ( Automatic Colt Pistol, also known as .32 Automatic) is a centerfire pistol cartridge. It is a semi-rimmed, straight-walled cartridge developed by firearms designer John Browning, initially for use in the FN M1900 semi-automatic pi ...
,
.380 ACP The .380 ACP (9×17mm) ( Automatic Colt Pistol) is a rimless, straight-walled pistol cartridge developed by firearms designer John Moses Browning. The cartridge headspaces on the mouth of the case.Wilson, R. K. ''Textbook of Automatic Pisto ...
,
9×18mm Makarov The 9×18mm Makarov (designated 9mm Makarov by the C.I.P. and often called 9×18mm PM) is a Soviet pistol and submachine gun cartridge. During the latter half of the 20th century it was a standard military pistol cartridge of the Soviet Union a ...
, etc.) There are exceptions such as the simple blowback pistols from
Hi-Point Firearms Hi-Point Firearms, also known as Strassell's Machine, Inc (distributed by MKS Supply), is an American firearms manufacturer based in Mansfield, Ohio. All of their firearms are manufactured in several different locations in Ohio. Construction of ...
which include models chambered in .40 S&W and
.45 ACP The .45 ACP ( Automatic Colt Pistol) or .45 Auto (11.43×23mm) is a rimless straight-walled handgun cartridge designed by John Moses Browning in 1904, for use in his prototype Colt semi-automatic pistol. After successful military trials, it ...
. Simple blowback operation can also be found in small-bore (such as
.22LR The .22 Long Rifle or simply .22 LR or 22 (metric designation: 5.6×15mmR) is a long-established variety of .22 caliber rimfire ammunition originating from the United States. It is used in a wide range of rifles, pistols, revolvers, smoothb ...
) semi-automatic rifles, carbines and submachine guns. Most simple blowback rifles are chambered for the
.22 Long Rifle The .22 Long Rifle or simply .22 LR or 22 (metric designation: 5.6×15mmR) is a long-established variety of .22 caliber rimfire ammunition originating from the United States. It is used in a wide range of rifles, pistols, revolvers, smo ...
cartridge. Popular examples include the
Marlin Model 60 The Marlin Model 60, also known as the Marlin Glenfield Model 60, is a semi-automatic rifle that fires the .22 LR rimfire cartridge. Produced by Remington Arms in Huntsville, Alabama formerly in Mayfield, Kentucky, formerly by Marlin Firearms Co ...
and the
Ruger 10/22 The Ruger 10/22 is a series of semi-automatic rifles produced by American firearm manufacturer Sturm, Ruger & Co., chambered for the .22 Long Rifle rimfire cartridge. It uses a patented 10-round rotary magazine, though higher capacity box ...
. Most blowback carbines and submachine guns are chambered for pistol cartridges such as the 9×19mm Parabellum, .40 S&W and .45 ACP. Examples include the MP 40,
Sten The STEN (or Sten gun) is a family of British submachine guns chambered in 9×19mm which were used extensively by British and Commonwealth forces throughout World War II and the Korean War. They had a simple design and very low production co ...
and
UZI The Uzi (; he, עוזי, Ūzi; officially cased as UZI) is a family of Israeli open-bolt, blowback-operated submachine guns and machine pistols first designed by Major Uziel "Uzi" Gal in the late 1940s, shortly after the establishment of the ...
. The bolt can be made bigger and more massive in these weapons than in handguns, as they are intrinsically heavier and designed, ideally at least, to be fired with both hands, often with the aid of a shoulder stock; and these factors help to ameliorate the disruption to the shooter's aim caused by the heavy bolt's movement. Consequently, simple blowback is adequate for somewhat more powerful rounds in submachine guns than in standard pistols. There were also a few rifles that chambered cartridges specifically designed for blowback operation. Examples include the
Winchester Model 1905 The Winchester Model 1905 (also known as the Model 05), is a blowback-operated, semi-automatic rifle produced by the Winchester Repeating Arms Company beginning in 1905 and discontinued in 1920. This rifle loads cartridges from a 5 or 10-round ...
,
1907 Events January * January 14 – 1907 Kingston earthquake: A 6.5 Moment magnitude scale, Mw earthquake in Kingston, Jamaica, kills between 800 and 1,000. February * February 11 – The French warship French cruiser Jean Bart ( ...
and
1910 Events January * January 13 – The first public radio broadcast takes place; live performances of the operas ''Cavalleria rusticana'' and ''Pagliacci'' are sent out over the airwaves, from the Metropolitan Opera House in New York Ci ...
. The only known assault rifle to use simple blowback was the Burton Model 1917. Whereas simple blowback is limited to guns using low-power rounds, it is so efficient in this context that in small-calibre
semi-automatic pistols A semi-automatic pistol is a type of repeating single- chamber handgun (pistol) that automatically cycles its action to insert the subsequent cartridge into the chamber (self-loading), but requires manual actuation of the trigger to actua ...
it is by now almost ubiquitous. Heavier calibre semiautomatic handguns typically employ a short recoil system, of which by far the most common type are Browning-derived designs which rely on a locking barrel and
slide Slide or Slides may refer to: Places * Slide, California, former name of Fortuna, California Arts, entertainment, and media Music Albums * ''Slide'' (Lisa Germano album), 1998 * ''Slide'' (George Clanton album), 2018 *''Slide'', by Patrick Glees ...
assembly instead of blowback. But blowback guns can be used to fire powerful cartridges if they are of the other two types already mentioned: API or delayed blowback.


Advanced primer ignition (API) blowback

In the API blowback design, the primer is ignited when the bolt is still moving forward and before the cartridge is fully chambered (akin to the fire- out-of-battery principle used in some mountain guns like Canon de 65 M (montagne) modele 1906, although there the bolt is locked and whole ordnance is moving at fire). This requires a very careful design to ensure the proper balance and equalization of forces between the projectile weight, propellant charge, barrel length, bolt weight, and return spring strength. In a simple blowback design, the propellant gases have to overcome static inertia to accelerate the bolt rearwards to open the breech. In an API blowback, they first have to do the work of overcoming forward momentum to arrest the forward motion of the bolt. Because the forward and rearward speeds of the bolt tend to be approximately the same, the API blowback allows the weight of the bolt to be halved. Because the momentum of the two opposed bolt motions cancels out over time, the API blowback design results in reduced recoil. According to Anthony G. Williams, the "API blowback principle is used virtually in all
open bolt A firearm is said to fire from an open bolt if, when ready to fire, the bolt and working parts are held to the rear of the receiver, with no round in the chamber. When the trigger is actuated, the bolt travels forward, feeds a cartridge from t ...
sub-machine gun A submachine gun (SMG) is a magazine-fed, automatic carbine designed to fire handgun cartridges. The term "submachine gun" was coined by John T. Thompson, the inventor of the Thompson submachine gun, to describe its design concept as an automat ...
s" (technically, although generally these are known as "simple blowback" weapons due to the very late ignition of the cartridge compared to specially designed API blowback guns like the MK 108), although "the relatively low pressures and velocities mean that extended chambers and rebated-rim cartridges are not required" for sub-machine guns.Anthony G. Williams, ''Rapid Fire'', Airlife UK 2000, page 65 An example of API in sub-machine guns is the L2A3
Sterling submachine gun The Sterling submachine gun is a British submachine gun (SMG). It was tested with the British Army in 1944–1945 as a replacement for the Sten but it did not start to replace it until 1953. A successful and reliable design, it remained as stan ...
, where the maximum chamber pressure is achieved while the breechblock is still moving forward and is about 0.46 mm away from the rear face of the chamber. In heavier weapons, advanced primer ignition (API) was originally developed by Reinhold Becker for use on the
Becker Type M2 20 mm cannon The Becker Type M2 20 mm cannon was a German autocannon developed for aircraft use during World War I by '' Stahlwerke Becker''. It was first mass-produced in 1916 and was installed in a variety of aircraft. It was the only German autocannon ...
. It became a feature of a wide range of designs that can be traced back to Becker's, including the Oerlikon cannon widely used as anti-aircraft weapons during World War II. To increase performance of API blowback firearms, larger calibre APIB guns such as the Becker and Oerlikon use extended chambers, longer than is necessary to contain the round, and ammunition for APIB firearms come with straight-sided cartridges with rebated rims (rims that are smaller in diameter than the cartridge itself).Anthony G. Williams, ''Rapid Fire'', Airlife UK 2000, pages 63-68 The last part of forward motion and the first part of the rearward motion of the case and bolt happen within the confines of this extended chamber. As long as the gas pressure in the barrel is high, the walls of the case remain supported and the breach sealed, although the case is sliding rearwards. This sliding motion of the case, while it is expanded by a high internal gas pressure, risks tearing it apart, and a common solution is to grease the ammunition to reduce the friction. The case needs to have a rebated rim because the front end of the bolt will enter the chamber, and the extractor claw hooked over the rim therefore has to fit also within the diameter of the chamber. The case generally has very little neck, because this remains unsupported during the firing cycle and is generally deformed; a strongly necked case would be likely to split. The API blowback design permits the use of more powerful ammunition in a lighter gun than would be achieved by using simple blowback, and the reduction of felt recoil results in further weight savings. The original Becker cannon, firing 20×70mmRB ammunition, was developed to be carried by World War I aircraft, and weighed only 30 kg.Anthony G. Williams, ''Flying Gun World War I'', Airlife UK 2003, pages 89-90 Oerlikon even produced an anti-tank rifle firing 20×110mmRB ammunition using the API blowback operation, the SSG36. On the other hand, because the design imposes a very close relationship between bolt mass, chamber length, spring strength, ammunition power and rate of fire, in APIB guns high rate of fire and high muzzle velocity tend to be mutually exclusive. API blowback guns also have to fire from an open bolt, which is not conducive to accuracy and also prevents synchronized fire through an aircraft propeller arc. According to a United States Army Materiel Command engineering course from 1970, "The advanced primer ignition gun is superior to the simple blowback because of its higher firing rate and lower recoil momentum. However, favorable performance depends on timing that must be precise. A slight delay in primer function, and the gun reverts to a simple blowback without the benefit of a massive bolt and stiffer driving spring to soften the recoil impact. ..The exacting requirements in design and construction of gun and ammunition reduce this type almost to the point of academic interest only." API mechanisms are very sensitive to the ammunition used. For example, when the Germans switched their MG FF (an Oerlikon FFF derivative) to their new, lighter mine shell, they had to rebalance the spring strength and bolt weight of the gun, resulting in a new MG FF/M model with ammunition not being interchangeable between the two models.Anthony G. Williams, ''Rapid Fire'', Airlife UK 2000, pages 65 and 166 The 30 mm MK 108 cannon was perhaps the apogee of API blowback technology during World War II. The principle is also used in some automatic grenade launchers, for example in the US
Mk 19 grenade launcher The Mk 19 grenade launcher (pronounced Mark 19) is an American 40 mm belt-fed automatic grenade launcher that was first developed during the Vietnam War. Overview The Mk 19 is a belt-fed, blowback-operated, air-cooled, crew-serve ...
or Russian
AGS-30 The AGS-30 ''Atlant'' is a Russian automatic grenade launcher currently in production in Russia and in service with the Russian armed forces. Description Designed on the basis of AGS-17, the AGS-30 provides better mobility, longer range and be ...
.


Delayed blowback

For more powerful rounds that cannot be safely used in simple blowback, or in order to obtain a lighter mechanism than the simple format can provide, the alternative to API is some system of ''delayed'' or ''retarded'' blowback, in which the bolt is never fully locked, but is initially held in place, sealing the cartridge in the chamber by the mechanical resistance of one of various designs of delaying mechanism. As with the resistance provided by momentum in API, it takes a fraction of a second for the propellant gases to overcome this and start moving cartridge and bolt backwards; this very brief delay is sufficient for the bullet to leave the muzzle and for the internal pressure in the barrel to decrease to a safe level. The bolt and cartridge are then pushed to the rear by the residual gas pressure. Because of high pressures, rifle-caliber delayed blowback firearms, such as the FAMAS and G3, typically have fluted chambers to ease extraction. Below are various forms of delayed-blowback actions:


Roller-delayed

Roller-delayed blowback was first used in Mauser's ''Gerät 06H'' prototype. Roller-delayed blowback operation differs from roller-locked recoil operation as seen in the
MG 42 The MG 42 (shortened from German: ''Maschinengewehr 42'', or "machine gun 42") is a German recoil-operated air-cooled general-purpose machine gun used extensively by the Wehrmacht and the Waffen-SS during the second half of World War II. Ente ...
and gas operated roller locked, as seen in the ''Gerät 03'' and ''Gerät 06''. Unlike the MG 42, in roller-delayed blowback the barrel is fixed and does not recoil, and unlike the ''Gerät 03'' and ''Gerät 06'' and StG 44, roller-delayed blowback systems lack a gas piston. These omissions are conducive to relatively light construction by significantly reducing the number of parts required and the amount of machining required to produce a rifle. As the bolt head is driven rearward, rollers on the sides of the bolt are driven inward against a tapered bolt carrier extension. This forces the bolt carrier rearward at a much greater velocity and delays movement of the bolt head. The primary advantage of roller-delayed blowback is the simplicity of the design compared to gas or recoil operation. The roller-delayed blowback firearm action was patented by Mauser's Wilhelm Stähle and
Ludwig Vorgrimler Ludwig Vorgrimler (September 7, 1912 – February 23, 1983) is the man most commonly associated with the design of the Spanish roller-delayed CETME rifle, and its prolific offspring from the German gunmaker Heckler & Koch such as the G3, HK ...
. Though appearing simple, its development during World War II was a hard technical and personal effort, as German engineering, mathematical and other scientists had to work together on a like-it-or-not basis led by Ott-Helmuth von Lossnitzer, the director of Mauser Werke's Weapons Research Institute and Weapons Development Group. Experiments showed roller-delayed blowback firearms exhibited bolt-bounce as the bolt opened at an extreme velocity of approximately during automatic fire. To counter bolt-bounce the perfect angle choice on the nose of the bolt head had to be found to significantly reduce the opening velocity of the bolt. The extremely high bolt carrier velocities problem was not solved by trial and error. Mathematician
Karl Maier Karl Maier (1901 – 25 July 2000) was a German Esperantist and member of the Universala Esperanto Asocio, for more than 50 years. Maier learned Esperanto in 1924. After emigration from Germany in 1930, he was living with Esperanto enthusiasts am ...
provided analysis of the components and assemblies in the development project. In December 1943 Maier came up with an equation that engineers used to change the angles in the receiver to 45° and 27° on the locking piece relative to the longitudinal axis reducing the bolt-bounce problem. With these angles the geometrical transmission ratio of the bolt carrier to the bolthead became 3:1, so the rear bolt carrier was forced to move 3 times faster than the bolthead. The rearward forces on the bolt carrier and receiver were 2:1. The force and impulse transmitted to the receiver increases with the force and impulse transmitted to the bolt carrier. Making the bolt carrier heavier lessens the recoil velocity. For Mausers
StG 45(M) The StG 45(M) (abbreviation of Sturmgewehr 45, "Assault Rifle 45") sometimes referred to as the MP 45(M), was a prototype assault rifle developed by Mauser for the '' Wehrmacht'' at the end of World War II, using an innovative roller-delayed bl ...
project Maier assumed a bolt head and bolt carrier (1 to 3 ratio). The prototype StG 45 (M) assault rifle had 18 longitudinal gas relief flutes cut in the
chamber Chamber or the chamber may refer to: In government and organizations *Chamber of commerce, an organization of business owners to promote commercial interests *Legislative chamber, in politics *Debate chamber, the space or room that houses deliber ...
wall to assist the bloated cartridge casing from the chamber walls during extraction. Fluting the end of the chamber provides pressure equalization between the front outer surface of the cartridge case and its interior and thus ensures extraction without tearing the case making extraction easier and more reliable. In 1944 other German companies like Großfuß (de), Rheinmetall and
C.G. Haenel C.G. Haenel is a German weapon manufacturer located in Suhl, Germany. History The Prussian commissioner for firearms manufacturing, Carl Gottlieb Haenel, began producing bicycles and weapons in 1840 — a combination which was not uncommon at t ...
showed interest in developing roller-delayed blowback small arms. Großfuß worked on a roller-delayed blowback
MG 45 MG, Mg, or mg and variants may refer to: Organizations * MG Cars, an automotive marque of the now defunct MG Car Company * MG Motor, a present-day car manufacturing company * MG JW Automobile, a Pakistani automobile manufacturer * Champion Air ( ...
general-purpose machine gun that, like the StG 45 (M), had not progressed beyond the prototype stage by the end of World War II. After World War II, former Mauser engineers Ludwig Vorgrimler and
Theodor Löffler Theodor is a masculine given name. It is a German form of Theodore. It is also a variant of Teodor. List of people with the given name Theodor * Theodor Adorno, (1903–1969), German philosopher * Theodor Aman, Romanian painter * Theodor Bluege ...
perfected the mechanism between 1946 and 1950 while working for the French small arms manufacturer Centre d'Etudes et d'Armament de Mulhouse (CEAM). In 1950 Ludwig Vorgrimler was recruited to work for
CETME CETME () is a Spanish government design and development establishment. While being involved in many projects CETME was mostly known for its small arms research and development. The CETME Model 58 and CETME Model L are its most notable project ...
in Spain. The first full-scale production rifle to utilize roller-delay was the Spanish CETME battle rifle, which was closely followed by the Swiss
SIG SG 510 The Sturmgewehr 57 is a selective fire battle rifle designed by Schweizerische Industrie Gesellschaft (now SAN Swiss Arms) of Switzerland. It uses a roller-delayed blowback system similar to the H&K G3 and CETME rifles. The AM 55 entered service ...
and the CETME Model B-based Heckler & Koch G3. The G3 bolt features an anti-bounce mechanism that prevents the bolt from bouncing off the barrel's breech surface.Woźniak, Ryszard. Encyklopedia najnowszej broni palnej – tom 2 G-Ł. Bellona. 2001. pp. 7–10. The G3's "bolt head locking lever" is a spring-loaded claw mounted on the bolt carrier that grabs the bolt head as the bolt carrier group goes into battery. The lever essentially ratchets into place with friction, providing enough resistance to being re-opened that the bolt carrier does not rebound. Due to the relative low
bolt thrust Bolt thrust or breech pressure is a term used in internal ballistics and firearms (whether small arms or artillery) that describes the amount of rearward force exerted by the propellant gases on the bolt or breech of a firearm action or breech wh ...
exhibited by pistol cartridges the anti-bounce mechanism is omitted by Heckler & Koch on their roller-delayed blowback firearms chambered for pistols cartridges. Heckler & Koch's
MP5 The Heckler & Koch MP5 (german: Maschinenpistole 5) is a 9x19mm Parabellum submachine gun, developed in the 1960s by a team of engineers from the German small arms manufacturer Heckler & Koch. There are over 100 variants and clones of the MP5, ...
submachine gun is the most common weapon still in service worldwide using this system. The Heckler & Koch P9 semi-automatic pistol,
CETME Ameli The Ameli (abbreviated from the Spanish ''Ametralladora ligera'' or "light machine gun") is a 5.56mm light machine gun designed for the Spanish Army (''Ejército de Tierra'') by the nationally owned and operated ''Centro de Estudios Técnicos de ...
light machine gun and
SIG MG 710-3 The SIG MG 710-3 is a Swiss 7.62 mm general-purpose machine gun (GPMG) designed and manufactured by SIG - Schweizerische Industrie Gesellschaft (currently Swiss Arms AG). The weapon was developed as a commercial venture primarily for export, s ...
and Heckler & Koch HK21 general-purpose machine guns also use it. Roller-delayed blowback arms are ammunition specific, since they lack an adjustable gas port or valve to adjust the arm to various propellant and projectile specific pressure behavior. Their reliable functioning is limited by specific ammunition and arm parameters like bullet weight, propellant charge, barrel length and amount of wear. At the moment of cartridge ignition the chamber has to be and remain sealed, until the bullet has exited the barrel and the gas pressure within the bore has dropped to a safe level before the seal is broken and chamber starts to open. For obtaining a proper and safe functioning parameters bandwidth arms manufactures offer a variety of locking pieces with different mass and shoulder angles and cylindrical rollers with different diameters. The angles are critical and determine the unlock timing and gas pressure drop management as the locking piece acts in unison with the bolt head carrier. The bolt gap width determines the headspace and hence the correct positioning of the cartridges in the (closed) chamber. Due to usage wear the bolt gap between the locking piece and bolt head carrier is expected to gradually increase. It can be determined and checked by a
feeler gauge A feeler gauge is a tool used to measure gap widths. Feeler gauges are mostly used in engineering to measure the clearance between two parts. Description They consist of a number of small lengths of steel of different thicknesses with measure ...
measurement and can be altered by changing the cylindrical rollers for rollers with a different diameter. Installing larger diameter rollers will increase the bolt gap and push the locking piece forward. Installing smaller diameter rollers results in the reverse effects.


Lever-delayed

Lever-delayed blowback utilizes leverage to put the bolt at a mechanical disadvantage, delaying the opening of the breech. When the cartridge pushes against the bolt face, the lever moves the bolt carrier rearward at an accelerated rate relative to the light bolt. Leverage can be applied with a dedicated part or through inclined surfaces interacting with each other. This leverage significantly increases resistance and slows the movement of the lightweight bolt. The reliable functioning of lever-delayed blowback arms is limited by specific ammunition and arm parameters like bullet weight, propellant charge, barrel length and amount of wear. John Pedersen patented one of the first known designs for a lever-delay system. The mechanism was also used by Hungarian arms designer
Pál Király Pál Király (1880–1965) was a Hungarian engineer and weapons designer. Work & designs He is best known for development of lever-delayed blowback firearms, and being employed by the Danuvia company in the 1930s and 1940s to design and pr ...
in the 1910s and 1930s and used in the Danuvia 39M and 43M submachine guns for the
Hungarian Army The Hungarian Ground Forces ( hu, Magyar Szárazföldi Haderő) is the land branch of the Hungarian Defence Forces, and is responsible for ground activities and troops including artillery, tanks, APCs, IFVs and ground support. Hungary's ground ...
. After World War II, Király settled in the Dominican Republic and developed the
Cristóbal Carbine The .30 Kiraly-Cristóbal Carbine, also known as the San Cristóbal or Cristóbal Automatic Rifle was manufactured by the Dominican Republic’s Armería San Cristóbal Weapon Factory. History and development Although called a carbine, the gun ma ...
(or Király-Cristóbal Carbine) employing a similar mechanism. Other weapons to use this system are the Hogue Avenger and
Benelli B76 The B76 is a pistol A pistol is a handgun, more specifically one with the chamber integral to its gun barrel, though in common usage the two terms are often used interchangeably. The English word was introduced in , when early handguns we ...
pistols, the
FNAB-43 The FNAB-43 is an Italian designed and developed submachine gun manufactured from 1943 to 1945. The first prototype was built in 1942 and the ~1,000 built by the FNA-B according to Ian McCollum of Forgotten Weapons (''Fabbrica Nazionale d'Armi di ...
submachine gun, the
TKB-517 The TKB-517 (russian: ТКБ-517) is an assault rifle designed by German Aleksandrovich Korobov. This rifle was externally similar to the AK-47, but based on the lever-delayed blowback mechanism invented by John Pedersen and refined by Pál ...
, VAHAN and FAMAS assault rifles, the Sterling 7.62 and AVB-7.62 battle rifles/light machine guns, and the
AA-52 The ''AA-52'' (full designation in French: ''Arme Automatique Transformable Modèle 1952'', "Transformable automatic weapon model 1952") is one of the first French-produced guns of the post–World War II era. It was manufactured by the French g ...
general-purpose machine gun.


Gas-delayed

Gas-delayed blowback should not be confused with gas-operation. The bolt is never locked, and so is pushed rearward by the expanding propellant gases, as in other blowback-based designs. However, propellant gases are vented from the barrel into a cylinder with a piston that delays the opening of the bolt. It was used by some World War II German designs for the
7.92×33mm Kurz The 7.92×33mm ''Kurz'' (designated as the 7.92 x 33 kurz by the C.I.P.) is a rimless bottlenecked intermediate rifle cartridge developed in Nazi Germany prior to and during World War II, specifically intended for development of the Sturmgeweh ...
cartridge, including the
Volkssturmgewehr The Volkssturmgewehr ("People's Storm - Rifle") is the name of several rifle designs developed by Nazi Germany during the last months of World War II. They share the common characteristic of being greatly simplified as an attempt to cope with se ...
rifle (with little effectiveness) and the
Grossfuss Sturmgewehr Grossfuss Sturmgewehr was a prototype assault rifle designed during World War II by Kurt Horn at the Grossfuss company ( Metall- und Lackwarenfabrik Johannes Großfuß) better known for their contribution to the German arsenal made with the MG 4 ...
(with slightly more efficiency), and after the war by the
Heckler & Koch P7 The Heckler & Koch P7 is a German 9×19mm semi-automatic pistol designed by Helmut Weldle and produced from 1979 to 2008 by Heckler & Koch GmbH (H&K). It was revealed to the public for the first time in 1976 as the PSP (''Polizei-Selbstlade-Pi ...
,
Walther CCP The Walther CCP (''Concealed Carry Pistol'') is a delayed blowback semi-automatic pistol developed by Carl Walther Sportwaffen for the concealed carry civilian market. Product evolution The Walther CCP was introduced in March 2014. It is availa ...
, Steyr GB and M-77B pistols.


Chamber-ring delayed

When a cartridge is fired, the case expands to seal the sides of the chamber. This seal prevents high-pressure gas from escaping into the action of the gun. Because a conventional chamber is slightly oversized, an unfired cartridge will enter freely. In a chamber-ring delayed firearm, the chamber is conventional in every respect except for a concave ring within the chamber wall. When the cartridge is fired, the case expands into this recessed ring and pushes the bolt face rearward. As the case moves to the rear this ring constricts the expanded portion of the case. The energy required to squeeze the walls of the cartridge case slows the rearward travel of the case and slide, reducing their mass requirements. The first known use of the system was on the Fritz Mann pistol in 1920 and later on the High Standard Corp model T3 experimental pistol developed by Ott-Helmuth von Lossnitzer while working for High Standard. Other firearms that used this system were the LWS
Seecamp L. W. Seecamp Co. was an American manufacturer of pocket pistols located in Milford, Connecticut from 1981 to 2014. In 2014, the company announced that Whalley Precision would take over production of the pistols. History L. W. Seecamp Co. was ...
pistol, the AMT Automag II, and the Kimball .30 Carbine pistol. The
SIG SG 510 The Sturmgewehr 57 is a selective fire battle rifle designed by Schweizerische Industrie Gesellschaft (now SAN Swiss Arms) of Switzerland. It uses a roller-delayed blowback system similar to the H&K G3 and CETME rifles. The AM 55 entered service ...
rifle family incorporates a chamber ring near the shoulder which is used to avoid bolt-bounce rather than a delay element.


Hesitation locked

John Pedersen's patented system incorporates a breech block independent of the slide or bolt carrier. When in battery, the breech block rests slightly forward of the locking shoulder located in the frame of the firearm. When the cartridge is fired, the cartridge case, bolt and slide move together a short distance until the breech block strikes the locking shoulder and stops. The slide continues rearward with the momentum it acquired in the initial phase while the breech remains locked. This allows chamber pressure to drop to safe levels once the bullet departs the barrel. The continuing motion of the slide lifts the breech block from its recess and pulls it rearward, continuing the firing cycle. The Pedersen
Remington Model 51 The Remington Model 51 is a small pocket pistol designed by John Pedersen and manufactured by Remington Arms in the early 20th century for the American civilian market. Remington manufactured approximately 65,000 Model 51 pistols in .32 ACP an ...
pistol, SIG MKMO submachine gun and R51 pistol are the only production firearms to have used this design.


Flywheel delayed blowback

Flywheel delayed blowback operation is where, during firing, the bolt opening is delayed by the rotational inertia of a flywheel. This is driven by a rack and pinion arrangement on the bolt carrier. The Barnitzke, Kazachok SMG, and the
MGD PM-9 The MGD PM-9 was a French open bolt submachine gun, designed in the late 1940s or early 1950s by Louis Debuit and manufactured in small numbers by French firm Merlin and Gerin in the 1950s. The PM9 was an unusual design in three different ways: ...
uses this operation.


Toggle-delayed

In toggle-delayed blowback firearms, the rearward motion of the breechblock must overcome significant mechanical leverage. The bolt is hinged in the middle, stationary at the rear end and nearly straight at rest. As the breech moves back under blowback power, the hinge joint moves upward. The leverage disadvantage keeps the breech from opening until the bullet has left the barrel and pressures have dropped to a safe level. This mechanism was used on the
Pedersen rifle The Pedersen Rifle, officially known in final form as the T1E3 rifle, was a United States semi-automatic rifle designed by John Pedersen that was made in small numbers for testing by the United States Army during the 1920s as part of a program to ...
and Schwarzlose MG M.07/12 machine gun.


Off-axis bolt travel

John Browning developed this simple method whereby the axis of bolt movement was not in line with that of the bore probably during late WWI and patented it in 1921. The result was that a small rearward movement of the bolt in relation to the
bore-axis The bore axis of a firearm is the longitudinal axis through the geometric center of the gun barrel. In a rifled barrel, the projectile (bullet/ball, pellet or slug) will spin around the bore axis as it goes through the barrel. Boresighting is a ...
required a greater movement along the axis of bolt movement, essentially magnifying the resistance of the bolt without increasing its mass. The French MAS-38 submachine gun of 1938 utilizes a bolt whose path of recoil is at an angle to the barrel. The
Jatimatic The Jatimatic is a Finnish 9×19mm Parabellum submachine gun developed in the late 1970s and early 1980s by Jali Timari. The submachine gun made its debut in 1983. The Jatimatic was manufactured in very limited numbers (approx. 400) initially by ...
and KRISS Vector use modified versions of this concept.


Radial-delayed

CMMG introduced the Mk 45 Guard rifle incorporating a radial-delay in 2017. This system uses the rotation of the bolt head to accelerate the bolt carrier of an
AR-15 An AR-15-style rifle is any lightweight semi-automatic rifle based on the Colt AR-15 design. The original ArmaLite AR-15 is a scaled-down derivative of Eugene Stoner's ArmaLite AR-10 design. The then Fairchild Engine and Airplane Corporatio ...
pattern rifle. The bolt locking lugs are adapted to incorporate 120° angles that rotate the bolt as it travels rearward under conventional blowback power. As the bolt rotates 22.5˚, it must accelerate the bolt carrier to the rear through an adapted 50° angle cam-pin slot. This acceleration amplifies the effective mass of the bolt carrier, slowing the speed of the bolt head. This delay allows pressure to drop prior to extraction without the penalty of a heavier bolt carrier assembly. The system is similar to roller and lever-delayed blowback in that it uses the mass of the bolt carrier moving at a faster rate than the bolt head to delay the action from opening. The design is described in .


Screw-delayed

First used on the Mannlicher Model 1893 automatic rifle, the bolt in screw-delayed blowback uses a turn bolt that was delayed by angled interrupted threads delayed by a quarter twist to unlock.
John T. Thompson John Taliaferro () (anglicized to "Tolliver"http://www.nfatoys.com/tsmg/web/genthomp.htm John T. Thompson; A Brief History) Thompson (December 31, 1860 – June 21, 1940) was a United States Army officer best remembered as the inventor of the Th ...
designed an autorifle that operated on a similar principle around 1920 and submitted it for trials with the US Army. This rifle, submitted multiple times, competed unsuccessfully against the
Pedersen Pedersen () is a Danish and Norwegian patronymic surname, literally meaning "son of Peder". It is the fourth most common surname in Denmark, shared by about 3.4% of the population, and the sixth most common in Norway. It is of similar origin as the ...
rifle and
Garand The M1 Garand or M1 rifleOfficially designated as U.S. rifle, caliber .30, M1, later simply called Rifle, Caliber .30, M1, also called US Rifle, Cal. .30, M1 is a semi-automatic rifle that was the service rifle of the U.S Army during World ...
primer-actuated rifle in early testing to replace the M1903 Springfield rifle. This operation is one of the most simple forms of delayed blowback but unless the ammunition is lubricated or uses a fluted chamber, the recoil can be volatile especially when using full length rifle rounds. Rotation of the bolt should be at least 90° to prevent ruptured cartridges. Another form of this operation using a helical screw to delay rearward movement was the
Salvator-Dormus M1893 The Salvator-Dormus M1893 also known as Skoda M1893 was a heavy machine gun of Austro-Hungarian origin. It was patented by Archduke Karl Salvator of Austria and Count George von Dormus and was manufactured by Skoda Works Plzeň. The Salvator-Dor ...
machine gun and later the prototype Kalashnikov Model 1942 submachine gun in 1942 and the Fox Wasp carbine.


Other blowback systems


Floating chamber

David Marshall Williams David Marshall Williams (November 13, 1900 – January 8, 1975) was an American firearms designer and convicted murderer who invented the floating chamber and the short-stroke piston. Both designs used the high-pressure gas generated in or near ...
(a noted designer for the U.S. Ordnance Office and later Winchester) developed a mechanism to allow firearms designed for full-sized cartridges to fire .22 caliber rimfire ammunition reliably. His system used a small "piston" that incorporates the chamber. When the cartridge is fired, the front of the floating chamber is thrust back by gas pressure impinging on the front of the chamber as in a traditional piston. This, added to the blowback energy imparted on the cartridge, pushes the bolt back with greater energy than either force alone. Often described as "accelerated blowback", this amplifies the otherwise anemic recoil energy of the .22 Rimfire cartridge. Williams designed a training version of the Browning machine gun and the
Colt Service Ace The Colt Ace or Colt Service Model Ace is a firearm that was designed to allow inexpensive and low-recoil practice while maintaining the feel of the military Model 1911 pistol. History While the Colt 1911 was chambered in the powerful .45 ACP car ...
.22 long rifle version of the
M1911 The M1911 (Colt 1911 or Colt Government) is a single-action, recoil-operated, semi-automatic pistol chambered for the .45 ACP cartridge. The pistol's formal U.S. military designation as of 1940 was ''Automatic Pistol, Caliber .45, M1911'' for t ...
using his system. The increased recoil produced by the floating chamber made these training guns behave more like their full-power counterparts while still using inexpensive low-power ammunition. The floating chamber is both a blowback and gas operated mechanism.


Primer actuated

Primer actuated firearms use the energy of
primer Primer may refer to: Arts, entertainment, and media Films * ''Primer'' (film), a 2004 feature film written and directed by Shane Carruth * ''Primer'' (video), a documentary about the funk band Living Colour Literature * Primer (textbook), a te ...
setback to unlock and cycle the firearm. John Garand developed the system in an unsuccessful bid to replace the M1903 bolt-action rifle in the early 1920s. Garand's prototypes worked well with US military .30-06 ammunition and uncrimped primers, but then the military changed from a fast burning gunpowder to a progressive burning Improved Military Rifle (IMR) powder. The slower pressure rise made the primer actuated prototypes unreliable, so Garand abandoned the design for a gas operated rifle that became the M1 Garand. AAI Corporation used a primer piston in a rifle submitted for the SPIW competition. Other rifles to use this system were the Postnikov APT and Clarke carbine as described in . A similar system is used in the spotting rifles on the
LAW 80 The LAW 80 (Light Anti-armour Weapon 80), regularly referred to as LAW 94 in British service, is a man-portable, disposable anti-tank weapon previously used by the British Army and a few other militaries. Description The weapon consists of an ext ...
and
Shoulder-launched Multipurpose Assault Weapon The Mk 153 Shoulder-Launched Multipurpose Assault Weapon (SMAW) is a smoothbore shoulder-fired rocket launcher. It is a portable assault weapon (i.e. bunker buster) and has a secondary anti-armor capability. Developed from the B-300, it was int ...
use a 9mm,
.308 Winchester The .308 Winchester is a smokeless powder rimless bottlenecked rifle cartridge widely used for hunting, target shooting, police, military, and personal protection applications globally. It is similar but not identical to the 7.62×51mm NATO ...
based cartridge with a .22 Hornet
blank cartridge A blank is a firearm cartridge that, when fired, does not shoot a projectile like a bullet or pellet, but generates a muzzle flash and an explosive sound ( muzzle report) like a normal gunshot would. Firearms may need to be modified to allow a b ...
in place of the primer. Upon firing, the Hornet case sets back a short distance, unlocking the action.


Case setback

The case cartridge itself has been used experimentally to actuate the action similar to Garand's primer-actuation. Known prototypes using this method of operation include two 1936 rifle designs, one by Mihail Mamontov and another by Makar Goryainov at TsKB-14, and a 1980s design by A.F. Barishev. The Mamontov and Goryainov rifles are only partially automatic; only the bolt unlocking is powered by the gases pushing the cartridge back, while the rest of the cycle (ejection, reloading) is done manually as in a traditional bolt-action rifle. A major problem with using the case cartridge as piston is that its motion is much faster (about 1 ms) compared to tapping gas further down the bore through a piston—about 5 ms in the Dragunov sniper rifle, which used the same cartridge as Mamontov's rifle. Barishev made a fully automatic, but rather bulky mechanism that used a mechanical delay. In his system, the case cartridge pushed back a tilting bolt face, that upon reaching a certain angle pushes backwards an unlocking lever that continues farther before unlocking the bolt. The GRAU however still gave a negative evaluation of Barishev's gun, pointing out that the main problems with reliability of firearms using the cartridge case as a piston were known since the 1930s and still unsolved.


Limited-utility designs


Blish lock

The Blish Lock is a breech locking mechanism designed by John Bell Blish based upon his observation that under extreme pressures, certain dissimilar metals will resist movement with a force greater than normal friction laws would predict. In modern engineering terminology, it is called static friction, or ''
stiction Stiction is the static friction that needs to be overcome to enable relative motion of stationary objects in contact. The term is a portmanteau of the words ''static'' and ''friction'', and is perhaps also influenced by the verb '' to stick''. Any ...
''. His locking mechanism was used in the
Thompson submachine gun The Thompson submachine gun (also known as the "Tommy Gun", "Chicago Typewriter", "Chicago Piano", “Trench Sweeper” or "Trench Broom") is a blowback-operated, air-cooled, magazine-fed selective-fire submachine gun, invented by United Sta ...
, Autorifle and Autocarbine designs. This dubious principle was later eliminated as redundant in the M1 and M1A1 versions of the submachine guns at the insistence of the US Army. Lubrication or fouling would completely defeat any delay. Whatever actual advantage a clean, unlubricated Blish system could impart could also be attained by adding a mere ounce of mass to the bolt.


Savage rotating barrel

The Savage system employed the theory that the rifling in the barrel caused a rotational force that would hold the gun locked until the projectile left the barrel. It was later discovered that the bullet had left the barrel long before any locking could occur. Savage pistols were in fact operating as simple blow back firearms. The French MAB PA-15 and PA-8 9mm pistols feature a similar design and work correctly.


Headspace actuated unlocking

An unusual operation that uses a bolt head that moves rearwards when firing that allows the cartridge to move backwards or even stretching it until the bolt unlocks. When firing the cartridge moves the bolt head rearwards around 2.5mm until it stops, then rotates the bolt to unlock and cycle the operation.


Magnet delay

An operation using a "simple blowback" type bolt that has neodymium magnets to delay its operation. A special buffer using this operation has been developed by TACCOM.


Other autoloading systems

Other autoloading systems are: *
Blow forward Blow-forward is a firearm action where the propellant gas pressure and the friction of the bullet traveling down the bore drag the whole gun barrel forward to facilitate the opening of the breech. This forward barrel motion provides most of t ...
, which is similar to blowback, but with the whole barrel being pushed forward rather than the bolt pushed back. * Recoil operation uses the rearward movement of parts of the weapon counter to the ejecta (bullet and propellant) moving forward, as described by Newton's third law of motion. * Gas-operated reloading


See also

*
List of blow forward firearms Below is the list of blow-forward weapons References {{Reflist Firearm actions blow forward Blow-forward is a firearm action where the propellant gas pressure and the friction of the bullet traveling down the bore drag the whole gun bar ...
*
List of delayed-blowback firearms Below is a list of delayed-blowback firearms. Lever delayed blowback Roller delayed blowback Gas delayed blowback Flywheel delayed blowback Hesitation lock Radial delayed blowback Toggle delayed blowback Vector delayed blowback ...
*
List of API blowback firearms This is a list of advanced primer ignition Blowback is a system of operation for self-loading firearms that obtains energy from the motion of the cartridge case as it is pushed to the rear by expanding gas created by the ignition of the propella ...


References


Bibliography

* Bremner, Derek, ''The MG42V and the Origins of Delayed Blowback Roller Lock: WWII German Equipment'' (Paperback). . *


External links


How Does it Work: Blowback Action
Forgotten Weapons
How Does it Work: Gas-Delayed Blowback
Forgotten Weapons
How Does It Work: Lever Delayed Blowback
Forgotten Weapons
How Does It Work: Roller Delayed Blowback
Forgotten Weapons
How Does It Work: Toggle Actions
Forgotten Weapons
Radial Delayed BlowbackRadial Delayed BlowbackInformation about the TZ45 submachine gun and the concept of advanced primer ignition
Animation and explanation at howstuffworks.com
Heckler and Koch USA now uses the "roller-delayed blowback" terminologyKewish primer actuated patent (1/4 assigned to Garand)US1603684
Garand patent making reference to earlier primer actuated application in 1919.
''Burke v US'', 67 F.Supp 827 (1947)
has comment about
Kewish Kewish is a Celtic surname of Manx origin. Background * Pronounced Q-ish. * A contraction of ''MacCuis'', '' McCuish'', ''MacKewsh'' or one of several variations of ''MacUais'', "The Noble's Son". Colla Uais was the 121st Milesian Monarch of I ...
and Garand.
What Does Blowback Mean In Airsoft?
{{DEFAULTSORT:Blowback (Firearms) Firearm actions Firearm terminology