Q: What does it mean when a firearm is fired from an open bolt?
A: The term “open bolt” indicates a firearm mechanism where the bolt remains in the open position until the trigger is pulled. Most semi-automatic firearms are of the more familiar “closed bolt” type. In such guns, the bolt is pulled to the rear, released and then moves forward, chambering a round, and then locks into battery before the trigger is pulled. When an open-bolt firearm is operated, the bolt is pulled to the rear and remains in that position until the trigger is pulled. Once the trigger is pulled, the bolt moves forward, chambers a round and then the gun fires when the bolt moves into battery. Open-bolt designs are generally used on full-automatic or selective-fire firearms, as the mechanism aids in cooling. The main downside is that the aim can be affected by the heavy bolt slamming forward at the moment of firing. However, this is not generally viewed as a significant problem when firing fully automatic firearms.
--Bruce N. Canfield