A semi or full 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. When the trigger is pulled the bolt goes forward, feeding a round from the magazine into the chamber and firing it. Like any other self-loading design without an external power supply, the action is cycled by the energy of the shot; this sends the bolt back to the rear, ejecting the empty cartridge case and preparing for the next shot. Generally, open bolt is used for automatic weapons and not for semi automatic weapons. Firearms using Advanced Primer Ignition blowback fire from open bolt only.
Read more about Open Bolt: Advantages, Disadvantages, Other Characteristics, Open-bolt Weapons, Mixed-mode Weapons, Legality
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