Blowback (arms)

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Blowback is a system in which automatic or semi-automatic firearms operate through the energy created by combustion in the chamber and bore acting directly on the bolt face through the cartridge. Other operating systems are recoil operation, gas-actuated, gatling and chain.

In the blowback system there is no positive lock between the bolt and the barrel. The mass of the bolt and force of its recoil spring act to keep the breech closed. The expanding gases from the fired round overcome this inertia and "blow back" the breech. The breech must be kept closed until the round has left the barrel and gas pressures have subsided. The weight of the bolt is the major factor in determining this, and to remain practical this system is only really useful for weapons using relatively low pressure rounds. Pure blowback operation is typically found only on semi-automatic small-caliber pistols and automatic submachine guns. There are also some low-velocity cannon or grenade launchers using blowback, derived from the MK 108. One of these is the Mk 19 grenade launcher.

For more powerful rounds and for lighter operating systems, some form of delayed or retarded blowback can be used, in which the bolt has to overcome some initial resistance while moving. There are various forms of delaying mechanism:

roller-delayed blowback, as in the HK G3
gas-delayed blowback, as in the HK P7
lever-delayed blowback, as in the FAMAS
Chamber-ring delayed blowback, as in the Seecamp pistol
Hesitation locked as in the Remington 51 pistol

While the above are effective delay mechanisms, there were some delaying systems which did not work as advertised. Two notable mechanisms are the Blish lock and Savage rotating barrel pistols. The Blish Lock used the dubious principle of dissimilar metal adhesion to delay the opening of the breech. It was eventually eliminated from the Thompson submachinegun as redundant. The Savage system theorized 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 and that Savage pistols merely operated on a pure blowback basis; the rotation of the barrel might play a minor role by slowing the rearward motion of the slide as the slide cycled.

Blowback can also refer to the combination of gasses, dirt, and debris (unburnt powder, metal shavings) that most firearms produce upon firing. This can cause great irritation to the eyes and many ranges or organizations suggest or require the use of safety glasses when firing pistols.

External links

[Blowback action], Animation and explanation at howstuffworks.com

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