Cooking Off - Machine Guns

Machine Guns

Cooking off is a characteristic of certain air-cooled machine guns firing from a closed bolt. In such a design, when the trigger is released the weapon feed leaves a final round in the chamber. Residual heat conducts through the cartridge case. If the kindling point of the propellant is eventually reached it will burn even though the primer has not been struck, thus firing the chambered round. Contrary to popular myth, this will not cause the machine gun to "runaway" at cyclic rate of fire (as compared to a Slamfire) because each chambered round has to first be brought up to temperature. The time this takes depends on the temperature of the chamber and of the environment, but is usually several seconds, although if caused deliberately may be very fast. During this time the barrel is cooling.

Cook offs in machine guns are prevented by:

• Cased ammunition: Among its many functions, the metallic cartridge case acts as a heat sink protecting the propellant from chamber heat. The case must first be brought up to temperature before the propellant inside can burn.
• Cooling: Barrels can be liquid-cooled (like a radiator in an automobile engine), or exchanged periodically. Most modern infantry machine guns (GPMG, General Purpose Machine Gun) are issued with several quick change barrels that are swapped out allowing one barrel to cool while the gun fires with the other.
• Open bolt: Most modern infantry machine guns (and submachine guns) fire from an open bolt, meaning the bolt remains to the rear when the trigger is released. Pulling the trigger releases the bolt forward and fires the weapon simultaneously. Assuming proper operation (no stoppages) a cook off is not possible with this design because a cartridge is not chambered until the moment the trigger is pulled and the weapon is fired, thus there is nothing in the chamber.
• Effective gunnery: Good gunners fire controlled bursts to ensure accuracy and limit overheating, and change barrels often.