bolt hold open

A fully automatic firearm is said to fire from an open bolt if, when ready to fire, the bolt and working parts are held to the rear of the receiver, with no round in the chamber. When the trigger is actuated, the bolt travels forward, feeds a cartridge from the magazine or belt into the chamber, and fires that cartridge in the same movement. Like any other self-loading design, the action is cycled by the gas expended from the round, hence the necessity of a gas system; this excess of gas sends the bolt back to the rear, ejecting the empty cartridge case and preparing for the next shot (or continuing forward again, if the trigger is held down in turn keeping the catch depressed and the weapon is an automatic). Generally, an open-bolt firing cycle is used for fully automatic weapons and not for semi-automatic weapons (except some semi-automatic conversions of automatic designs). Firearms using advanced primer ignition blowback inherently fire from open bolt only.

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