Cooking off

Cooking off (or thermally induced firing) is ammunition exploding prematurely due to heat in the surrounding environment.

A fast cook-off is a cook-off caused by fire. A slow cook-off is caused by a sustained thermal event less intense than fire.

A cooked-off round may cause a sympathetic detonation of adjacent rounds.

Inherent design flaws in early 17th century Swedish leather cannons led to the gun tube overheating which prematurely ignited the gunpowder, injuring the loader.

After the cooking off of artillery shells in the G5 howitzers in the late 1980s, the South African Army changed commands from "cease fire" to "cease loading". This allowed crews to fire any loaded shells to prevent them from heating up and exploding.

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.

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