(1) In ammunition, a leakage of gas rearward between the case and the chamber wall from the mouth of the case.
(2) In firearms, a design found in semiautomatic and automatic firearms where the weight of the bolt, the inertia of some component (typically the slide) supplemented with a recoil spring, and/or a mechanical delay is the main “locking” force. The action remains closed during the moment of firing until the chamber pressures have lowered to a safe level after which recoil forces cycle the firearm action. No mechanical locking of the breech occurs. Refer to Blowback, Delayed and Blowback, Simple.
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