Open bolt

A semi or 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. 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 (or continuing forward again, if the trigger is held down 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.

Compared to a closed-bolt design, open-bolt weapons generally have fewer moving parts. The firing pin is often part of the bolt, saving on manufacturing costs; the inertia of the bolt closing also causes the fixed firing pin to strike a blow on the primer, without need for a separate hammer/striker and spring. In automatic weapons an open bolt helps eliminate the dangerous phenomenon known as "cook-off", wherein the firing chamber becomes so hot that rounds spontaneously fire without trigger input, continuing to fire until the ammunition is exhausted. Open-bolt designs typically operate much cooler than closed-bolt designs due to the airflow allowed into the chamber, action and barrel during pauses between bursts, making them more suitable for constant full-automatic weapons such as machine guns.

The weapon is more prone to fire when dropped, and the open mechanism is more subject to picking up dirt when in the ready position, and so may require an additional ejector door or similar mechanism to exclude dust and dirt. Some open-bolt designs can suffer from a condition in which bolt retention fails and the weapon discharges even with no trigger input. Open-bolt machine guns could not be synchronized to fire through the arc of a propeller due to the relatively long period between sear disengagement and the ignition of the cartridge, making them harder to use as forward-firing weapons on tractor configuration fighter aircraft. Accuracy can suffer somewhat in an open-bolt design, again due to the longer delay between pulling the trigger and primer ignition, but this is generally less of a concern in automatic weapons, since after the first shot, the operation is indistinguishable from a firearm that fires from a closed bolt. It is only when the trigger is released that the bolt is caught in the rearward position; during automatic fire, both types bolts are free to operate at full rate, without hindrance. As most open-bolt guns are automatics, generally only weapons, such as submachine guns, converted to semi-automatic operation suffer from this.

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Wikipedia