Most modern civilian firearms fire from a closed-bolt position, where the round is fully seated into the barrel and the weapon’s bolt is forward and locked into the action. Firing from a closed-bolt position increases the accuracy of a firearm because there is no excessive movement of the bolt before the shot. The downside of this type of action is that during a sustained series of fire that generates a lot of heat, the bolt can get hot enough to inadvertently fire, or “cook-off,” a chambered round. This is why some firearms, particularly machine guns, fire from an open-bolt position.