Blowback (arms)

Blowback is one particular system in which an automatic or semi-automatic firearm may operate. (The main other systems are recoil-actuated, gas-actuated, gatling and chain.)

Related Topics:
Recoil-actuated - Gas-actuated - Gatling - Chain

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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. 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.

Related Topics:
Pistol - Submachine gun - Cannon - MK 108

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One of these is the Mk 19 grenade launcher.

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

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• roller-delayed blowback, as in the HK G3
• gas-delayed blowback, as in the HK P7
• level delayed blowback, as in the FAMAS