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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.
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. 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.