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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 gases created by the ignition of the propellant charge.[1]

Several types of blowback systems exist within this broad principle of operation, each distinguished by the level of energy derived through the blowback principle and the methods used to control bolt movement. In most actions that use blowback operation, the breech is not locked mechanically at the time of firing: the inertia of the bolt and recoil spring(s), relative to the weight of the bullet, delays opening of the breech until the bullet has left the barrel.[2] A few locked breech designs use a form of blowback (example: primer actuation) to perform the unlocking function.

Other operating principles for self-loading firearms include blow forward, gas operation, recoil operation, Gatling, and chain. The blowback principle may be considered a simplified form of gas operation, since the cartridge case behaves like a piston driven by the powder gases.[1]

Principle of operationEdit

The blowback system is generally defined as an operating system in which energy to operate the firearm's various mechanisms and provide automation is derived from the movement of the spent cartridge case pushed out of the chamber by rapidly expanding powder gases.[3] This rearward thrust, imparted against the bolt, is a direct reaction of the total reaction to the forward thrust applied to the bullet and the expansion of propellant gases.[3] Certain guns will use energy from blowback to perform the entire operating cycle (these are typically designs using relatively "low power" ammunition) 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.[3]

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 powder pressure in the chamber will undergo a degree of blowback action.[3] The energy from the expansion of gases on 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.[1] It is with how the movement of the bolt is controlled 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, advanced primer ignition and delayed blowback or retarded blowback.

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 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."[4]

Simple blowbackEdit


The simple (sometimes referred to as the "straight" or "pure") blowback system represents the most basic form of blowback operation and demonstrates the basic principles involved in the blowback cycle.[1] The simple blowback mechanism typically consists of the bolt which rests against the base of the cartridge case and a recoil spring that is compressed by the kinetic energy of the bolt when it is thrown back in recoil. The stored energy of the compressed spring then drives the bolt back forward into firing position.[5]

The cycle begins when the cartridge is fired. With an open-bolt cycle, the bolt is held by the trigger sear to the rear and the recoil spring is fully compressed. Pulling the trigger releases the sear, the action spring then propels the bolt forward, which strips a round from the feed system along the way. The bolt carries a new cartridge into the chamber with considerable velocity and at the end of its travel the firing pin fires the primer, igniting the propellent. The pressure of expanding gases from the propellant sends the projectile down the barrel and at the same time applies an opposite, rearward, bolt thrust force to the shell case against the breech face of the bolt, accelerating the bolt and casing rearward with a force equal to F = ma, where "m" is the mass of the bolt and casing, and "a" is the rate of acceleration of the bolt (the resistance of the recoil spring can be considered to be negligible until the bore pressure drops). The force is also equal to F = PA, where "P" is the instantaneous gas pressure inside the bore, and "A" is the cross-sectional area of the chamber (the pressure force and inertia force are equal and opposite, i.e. same "F" but in opposite directions). The breech is kept sealed by the internal pressure of the cartridge case against the chamber until the bullet has left the barrel; the inertia of the bolt mass ensures this (mass of the bolt + recoil spring, in some cases the hammer force too).[5] At this point the bore pressure is zero and the force driving the bolt back is also zero, but the case and bolt continue to the rear on their own momentum.[5] At the moment the bullet leaves the barrel, the momentum of the bullet and the rearward travelling bolt are equal and opposite, assuming a same diameter bore and chamber, such as the sten gun (which is not true with delayed blowback systems in which some of the momentum is initially transferred directly to the bulk of the gun, or with necked cartridges where the casing is a larger diameter than the projectile). The momentum of the bolt is gradually transferred to the body of the gun and the shooter's body as the recoil spring is compressed. As the bolt travels back, the spent cartridge case is extracted and ejected, and the firing mechanism is cocked by the rearward travelling bolt. The bolt eventually reaches a velocity of zero and the kinetic energy from the recoil impulse is now stored in the fully compressed spring (some energy loss does occur due to friction and the extraction and ejection sequences).[5][5] The cycle repeats until the last round is expended or the trigger is released engaging the sear to hold the bolt in the rear (open-bolt) position.

To remain practical, this system is only suitable for firearms using relatively low pressure cartridges. Pure blowback operation is typically found on semi-automatic, small-caliber pistols, small-bore semi-automatic rifles and submachine guns. Some low-velocity cannon and grenade launchers such as the Mk 19 grenade launcher also use blowback operation.

The barrel of a blowback pistol is generally fixed to the frame and the slide is held against the barrel only by the recoil spring tension. The slide starts to move rearward immediately upon ignition of the primer. As the cartridge moves rearward with the slide, it is extracted from the chamber and typically ejected clear of the firearm. The mass of the slide must be sufficient to hold the breech closed until the bullet exits the barrel and residual pressure is vented from the bore. A cartridge with too high a pressure or a slide with too little mass may cause the cartridge case to extract early, causing a separation or rupture. This generally limits blowback pistol designs to calibers less powerful than 9x19mm Parabellum (e.g., .25 ACP, .32 ACP, .380 ACP, 9x18mm Makarov, etc.). Any larger and the slide mass starts to become excessive, and therefore few blowback handguns in such calibers exist (see Recoil operation (Short recoil operation) for the method most commonly used by these pistols); the most notable exceptions are simple, inexpensive guns such as those made by Hi-Point Firearms which includes models chambered in .45 ACP, .40 S&W, .380 ACP and 9x19mm Parabellum.[6]

Most simple blowback rifles are chambered for the .22 Long Rifle cartridge. Popular examples include the Marlin Model 60 and the Ruger 10/22. Some blowback rifles or carbines are chambered for pistol cartridges, such as the 9mm Parabellum, .40 S&W and .45 ACP. Examples include the MP-40 and UZI submachine guns. There were also a few rifles that chambered cartridges specifically designed for blowback operation. Examples include the Winchester Model 1905, 1907 and 1910. A very unusual blowback firearm was created by fitting the M1903 Springfield rifle with a mechanism called the Pedersen device which retrofited a semi-automatic repeating action in the space normally occupied by the bolt of the firearm.

Advanced Primer Ignition (API) blowbackEdit

Template:Stack In the API blowback design, the primer is ignited when the bolt is still moving forward before the cartridge is fully chambered Image. In a plain blowback design, the propellant gases have to overcome static inertia to accelerate the bolt rearwards to open the breech. In an API blowback, they also have to do the work of overcoming forward momentum to stop 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.[7] 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 sub-machine guns", although "the relatively low pressures and velocities mean that extended chambers and rebated-rim cartridges are not required" for sub-machine guns.[8] In heavier weapons, advanced primer ignition (API) was originally developed by Reinhold Becker[9] for use on the Becker 20-mm automatic cannon. 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 WWII.[8]

To increase performance of API blowback firearms,[7] larger calibre APIB guns such as the Becker and Oerlikon use extended chambers, longer than is necessary to contain the round, ammunition for APIB firearms come with straight-sided cartridges with rebated rims (the rear of the cartridge case is smaller in diameter than the front).[10] 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 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 that would be achieved by using plain blowback, and the reduction of felt recoil results in further weight savings. The original Becker cannon, firing 20x70RB ammunition, was developed to be carried by WWI aircraft, and weighed only 30 kg.[11] Oerlikon even produced an anti-tank rifle firing 20x110RB 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.[10] API blowback guns also have to fire from an open bolt, which is not conducive to accuracy and means they can't be synchronized to fire through a propeller.

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 Minengeschoß shell, they had to rebalance the spring strength and bolt weight of the gun, resulting a new MG FF/M model with ammunition not being interchangeable between the two models.[10] The 30 mm MK 108 cannon was perhaps the apogee of API blowback technology during WWII.

According to an 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."[12]

An example of API in sub-machine guns is the L2A3 Sterling submachine gun, 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.[13] The principle is also used in some automatic grenade launchers, for example in the US Mk 19 grenade launcher or Russian AGS-30.

Delayed blowbackEdit

For more powerful rounds or for a lighter operating mechanism, some system of delayed or retarded blowback is often used, requiring the bolt to overcome some initial resistance while not fully locked. Because of high pressures, rifle-caliber delayed blowback firearms, such as the FAMAS and G3, typically have fluted chambers to ease extraction.

Other autoloading systemsEdit

Other autoloading systems are:

  • Blow forward where the barrel is the only moving component of the weapon that is dragged forward by the friction of the bullet until it leaves the barrel.
  • Recoil operation uses the rearward movement of parts of the weapon counter to the ejecta moving forward, as described by Newton's third law of motion.
  • Gatling and other mechanical means utilize mechanical energy from an operator turning a crank.
  • Chain and others utilize external power through electrical or hydraulic energy for operation.
  • Gas-operated reloading

See alsoEdit


  1. 1.0 1.1 1.2 1.3 Chinn, George M.: The Machine Gun, Volume IV: Design Analysis of Automatic Firing Mechanisms and Related Components, p. 3. Bureau of Ordnance, Department of the Navy, 1955.
  2. Walter H. B. Smith, Rifles, Military Service Publishing Co., 1948, "blowback semiautomatic operation" pp.88-89.
  3. 3.0 3.1 3.2 3.3 Chinn, 11.
  4. Chinn, p. 3
  5. 5.0 5.1 5.2 5.3 5.4 Chinn, 12.
  6. Template:Cite web
  7. 7.0 7.1 Chinn, page 31.
  8. 8.0 8.1 Anthony G. Williams, Rapid Fire, Airlife UK 2000, page 65
  9. Williams, Anthony G., Of Oerlikons and other things…… article
  10. 10.0 10.1 10.2 Anthony G. Williams, Rapid Fire, Airlife UK 2000, pages 63-68
  11. Anthony G. Williams, Flying Gun World War I, Airlife UK 2003, pages 89-90
  12. Automatic Weapons, AMC pamphlet no. 706-260, February 1970, page 2-47
  13. Template:Cite book


External linksEdit

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