Open bolt

Like any other self-loading design, the action is cycled by the energy released from the propellant, which sends the bolt back to the rear, compressing the mainspring in readiness for firing the next round.

Open-bolt designs typically remain much cooler in operation than closed-bolt types due to the airflow allowed into the chamber, action and barrel during pauses between bursts; moreover, unlike in the case of the closed-bolt format, the initial round in a burst is not introduced into the chamber until the moment before firing, and is thus only exposed to the residual heat for a fraction of a second.

These two features combine to make open-bolt operation more suitable for weapons such as machine guns, which are intended to be capable of prolonged automatic fire.

Firstly, the bolt retention mechanism may fail, resulting in a spontaneous discharge (i.e., without prior trigger input), with potentially dangerous consequences.

Some simple submachine gun designs, such as the Sten, can discharge spontaneously when dropped onto a hard surface – even when uncocked – as the collision can jolt the bolt backward far enough that on returning it will pick up a round from the magazine, chamber it and fire it; the risk is intrinsic to hand-held open-bolt guns unless safety features are included in the design.

Another shortcoming of the open-bolt principle is that there is a brief delay between the trigger-pull and the firing of the cartridge because the (rather inert) bolt has to move forward a significant distance between the two events.

The issue was most problematic in the use of forward-firing open-bolt machine guns and autocannons in (tractor configuration single-engine) fighters during the piston engine era.

Given the highly dynamic nature of aerial combat, the aforementioned intrinsic firing delay of open-bolt guns is particularly undesirable.

For handheld weapons, typically a lower rate of fire is desirable, as this will conserve ammunition and help keep the level of recoil more manageable.

This is not generally true, however, as open-bolt weapons send the bolt carrier back into a cocked position via the excess gas after the last round is fired.

A closed bolt requires the second step of cycling the action to remove the last round in the chamber (unless the weapon features an automatic hold-open device).

In this circumstance there will be a round in the chamber and a firing pin pressing on it with some force, but not enough to ignite the primer, which requires a sharp, focused impact.