Thermally-induced firing, more commonly known as cooking off, is ammunition exploding prematurely due to heat in the surrounding environment. Cooking off is most common in closed bolt machine guns or assault rifles and sometimes caseless firearms, and sometimes, bullets cooking off can cause a sympathetic detonation (i.e. a detonation of an explosive charge by a nearby explosion, usually unintended) of adjacent rounds, causing the weapon to fire ceaselessly until ammunition is completely expended. Cook-offs are usually caused by very hot chambers after fully automatic firing, where the cartridge can absorb enough energy and cause cooking off.
- Fast cook-off
Caused by fire.
- Slow cook-off
Caused by a sustained thermal event of less intensity than fire.
Cook-offs can be prevented through these methods:
- Using cased ammunition
The casings act as a heat sink to absorb the heat when the weapon is fired.
Barrels can be liquid-cooled or air-cooled or, in the case of machine guns, exchanged with the use of quick-change barrels.
- Open bolt
Most modern submachine guns and machine guns fire from open bolts (i.e. the bolt is held to the rear of the weapon until the trigger is pulled, where the bolt slams forward and fires the weapon). Cook-offs are not possible in open bolt firearms as a cartridge is not chambered until the trigger is pulled, which leads to there being nothing in the chamber.
- Effective gunnery
Good gunners usually fire in bursts to allow heat to dissipate after firing, potentially preventing cook-offs.