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An automatic or semiautomatic action that utilizes the expanding gases of the ignited propellant powder to drive the breechblock action rearward. In other firearms the breechblock holds the breech closed under the force of the recoil spring, no mechanical locking system being employed. In a blowback action, the inertia of the movable parts is such that the bullet leaves the muzzle after discharge before the breechblock moves rearward.