Closed bolt

A semi or full-automatic firearm which is said to fire from a closed bolt or closed breech is one where, when ready to fire, a round is in the chamber and the bolt and working parts are forward in battery.

When the trigger is pulled, the firing pin or striker fires the round; the action is cycled by the energy of the shot, sending the bolt to the rear, which extracts and ejects the empty cartridge case; and the bolt goes forward, feeding a fresh round from the magazine into the chamber, ready for the next shot.

In contrast, open-bolt designs are more often used in automatic weapons, such as machine guns.

For fast-firing automatic weapons, heat will rapidly build up from sustained firing, but accuracy is of less importance.

Thus, the improved heat dissipation of open-bolt designs is generally more desirable in automatic weapons.