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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.
Compared to a closed-bolt design, open-bolt weapons generally have fewer moving parts. The firing pin is usually part of the bolt, saving on manufacturing costs. 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. Open-bolt designs typically operate much cooler than closed-bolt designs, 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, making them harder to use as forward-firing weapons on tractor configuration fighter aircraft. Accuracy can suffer somewhat in an open-bolt design, but this is generally less of a concern in automatic weapons.
Many movies and video games portray open-bolt weapons as needing to be charged after reloading. This is not generally true however, as the operation of basic open bolt weapons sends the bolt carrier back into a cocked position via the excess gas from the spent round. The sole exception is if the trigger was held down after the last round has been fired, at which point the bolt will fly forward once more and stay there. In this case, the bolt merely needs to be retracted, and does not go forward as is sometimes portrayed.
Another feature of open-bolt designs is that the magazine simply needs to be removed to completely unload the weapon. A closed bolt requires the second step of cycling the action to remove the last round in the chamber. It is essential to remove a loaded magazine before performing maintenance, or trying to cycle or close the bolt (as is often done to keep the weapon clean when not in use). If one were to close the bolt (say by pulling the trigger and riding the bolt to the closed position), as soon as the bolt closes it will fire if a loaded magazine was left in the gun.
In the U.S., the BATF made a ruling in 1982 that semi-automatic open-bolt weapons are readily convertible to fully automatic fire, therefore such weapons manufactured after the date of this ruling are classed and controlled as fully automatic weapons (weapons manufactured prior to the ruling are grandfathered and are still considered semi-automatic).