Semi-automatic firearm

A semi-automatic, or self-loading, firearm is one that performs all steps necessary to prepare it to discharge again after firing—assuming cartridges remain in the firearm's feed device. Typically, this includes extracting and ejecting the spent cartridge case from the firing chamber, re-cocking the firing mechanism, and loading a new cartridge into the firing chamber. Although machine guns and selective-fire firearms do the same tasks, semi-automatic firearms do not automatically fire an additional round until the trigger is released and re-pressed by the person discharging it. That is, a semi-automatic firearm fires only one round (bullet) each time the trigger is pulled, and these firearms incorporate a "disconnector" to ensure that only one round is discharged for each pull of the trigger (releasing the trigger allows the sear to be reset, thereby preparing the firearm for the next shot; should the trigger not be released, the sear will not be reset; the "disconnector" ensures this behavior).

While all basic firearm actions require the action to be cycled manually before the first shot, semi-automatic, as well as automatic and selective-fire actions, are differentiated from other forms such as single-action or double-action revolvers, pump-action, bolt-action, or lever-action firearms by eliminating the need to manually cycle it after each shot. For example, to fire ten rounds from a semi-automatic or a selective-fire firearm set to fire semi-automatically, the action would initially be cycled to load the first round and the trigger would need to be pulled ten times (once for each round fired). For the other forms, the firearm's mechanism would require cycling manually prior to firing the next round. An automatic or a selective-fire firearm set to fire automatically would be able to discharge continuously as long as the trigger is held until the magazine or feed device runs out of ammunition. An automatic or a selective-fire firearm may incorporate an additional mechanism by which an otherwise automatic operation may be limited to a specific number of rounds in a "burst", sometimes three rounds when set to fire in bursts; in such a case, the trigger must be released, as in a semi-automatic firearm, to reset the sear for the next "burst". Obviously, this additional feature complicates the design of the sear and "disconnector", as well as the fire-control selector.

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