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Open bolt
From Wikipedia, the free encyclopedia
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.
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, often causing the weapon to fire uncontrollably until it jams or exhausts its ammunition. Accuracy can suffer somewhat in an open-bolt design, but this is generally less of a concern in automatic weapons.
Many open bolt weapons are shown, in movies and video games, to erroneously need 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.
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In the U.S., the ATF 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).