- Cooking off
Cooking off (or thermally induced firing) refers to ammunition exploding prematurely due to heat in the surrounding environment. It can also refer to a technique used when throwing grenades to achieve a controlled, predictable explosion.
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.
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 flash 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 because each chambered round has to first be brought up to temperature, which takes at least ten seconds or longer, and the barrel is cooling during this period.
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 two 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 preventing the weapon from overheating.
Most modern infantry assault rifles fire from a closed bolt, meaning that when ready to fire, there is a round in the chamber of the barrel and the bolt and working parts are in the forward position, closing the breech. Squeezing the trigger releases the striker or hammer, firing the cartridge in the chamber. Assuming proper operation (no stoppages) a cook off is possible with this design because a cartridge is kept chambered in the potentially hot chamber, where it can absorb enough heat to cause ignition of its propellant.
Caseless ammunition eliminates the metal case that typically holds the primer or igniter and the powder charge (smokeless powder) that propels the bullet. The metal case absorbs a large portion of the waste heat of firing. Ejecting this hot, empty case removes that heat from the weapon. With caseless rounds, other means of reducing waste heat are necessary, especially in automatic fire.
Cooking off is a serious hazard to crews in damaged and disabled tanks. Attempted solutions include storing ammunition under water and insulating ammunition compartments. The current technique, used in tanks like the M1 Abrams, is to armor the compartments and provide blow-off panels to channel the force of the explosion to the exterior of the tank and prevent the Jack-in-the-box effect.
Common terminology differs in definition for hand grenades; here, cooking off refers to the act of removing both the safety pin as well as allowing the safety level (spoon) to separate, thus priming the hand grenade, and holding on to it while momentarily allowing its fuze to partially burn down. The grenade thus detonates sooner after being thrown, limiting the time for the target(s) to take cover, escape the blast radius, or throw it back. This method is greatly discouraged among armed forces as the fuze length is unpredictable to a degree, posing a significant threat to the user and any nearby friendly personnel, should detonation occur earlier than expected.
Missiles and air-dropped bombs
The risk of aircraft armament cooking off is a significant hazard during pre-flight operations, especially for aircraft carriers. Fuel fires, which can spread across the flight deck rapidly and engulf many aircraft, are the most serious risk. This was a significant contributor to the 1967 fire disaster aboard the USS Forrestal, when such a fire (set off by an inadvertently fired Zuni rocket's striking the fuel tanks of a waiting A-4 Skyhawk) detonated two iron bombs of Korean War vintage which had been loaded onto the stricken bomber, rupturing the fuel tanks of adjacent aircraft and setting off a chain reaction of similarly cooked off bombs.
- Sympathetic detonation
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