A semi or fully-automatic firearm which is said to fire from an open bolt is one where, 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. The action is cycled by the energy of the shot sending the bolt back to the rear and ejecting the empty cartridge case ready for the next shot.

Comparison with closed bolt design

Pros

• Generally simpler to manufacture.
• Open chamber speeds cooling between bursts.

Cons

• Less accurate:
  • Movement of working parts inhibits accuracy.
  • Round does not seat consistently in the chamber.
• Not possible to lock the action forward e.g. in a suppressed weapon.

Open bolt weapons

• Uzi
• Sten
• Sterling

Mixed mode weapons

• FG42

Notes

In the U.S., the ATF have a policy whereby they consider most semi-automatic open bolt weapons to be easily convertible to fully-automatic fire, therefore they are classed and controlled as fully-automatic weapons.

See also

• Closed bolt