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Blowback is one of the primary methods of constructing a self-loading firearm action. In a blowback weapon, the energy that operates the action comes from the motion of the cartridge case as it is forced rearwards by expanding gases created by the ignition of the propellant charge. This effectively uses the cartridge case as the head of a piston: it can be considered a simplistic form of gas operation, though it is treated as separate for classification purposes.

Principle of operationEdit

Simple blowbackEdit

In a simple blowback weapon, the casing pushes back against an unlocked bolt: the primary concern is ensuring the weight of the operating components and the tension of the operating spring are enough to delay the opening of the breech until after the bullet has left the barrel. Simple blowback can be used with a closed or open bolt.

Advanced Primer IgnitionEdit

API blowback is an open-bolt operating system where the round's primer is struck before it has been fully chambered, meaning that some of the recoil force is offset by the continued forward motion of the bolt group. Many open-bolt blowback submachine guns technically use this method, but since their primer ignition occurs very close to the action entering battery, the effect on their operation is minimal.

"True" API blowback guns tend to have an extended chamber to properly contain the forces involved: such a system allows for either a reduction in the weight of the weapon's bolt, or for a reduction in felt recoil if the weight of the bolt remains the same. This system was initially developed for the Becker Type M2, since simple blowback is unsuited to large weapons: the weight of the bolt required to keep it closed in the initial milliseconds of firing renders such a weapon impractical. The Becker, for example, would have required a bolt weighing a quarter of a ton to function as a simple blowback weapon, so heavy that the return spring would either have to be so powerful the gun could not cycle, or too light for the weapon to close its own bolt with the barrel elevated.

Delayed blowbackEdit

Delayed or retarded blowback refers to systems requiring the bolt to overcome some form of initial mechanical resistance while not fully locked, as opposed to only the bolt's inertia and the pressure of the spring.