blowback

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 propellant charge.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 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, 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, 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, gas operation, and recoil operation.

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