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Premature explosion of ammunition

Cooking off (or thermally induced firing) is unfired weapon ammunition exploding prematurely due to heat in the surrounding environment. The term is used both for detonation of ammunition not loaded into a weapon, and unintended firing of a loaded weapon due to heating. 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. Insensitive munitions are designed to be less vulnerable to accidental firing induced by external heat.

Inherent design flaws in early 17th century Swedish leather cannons led to the gun tube overheating which prematurely ignited the gunpowder, injuring the loader. Muzzle loading cannon on merchant and naval vessels of the Age of Sail would fire if the vessels caught fire while the guns were loaded. Examples include the merchantman Earl Fitzwilliam and
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.

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. Apart from the possibility to cook off the heated propellant requires a special formula to allow for consistent muzzle velocity throughout all temperatures.

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 such as 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.

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 Forrestal, when such a fire (set off by an inadvertently fired Zuni rocket striking the fuel tanks of a waiting A 4 Skyhawk) detonated two unguided 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. Because of the age and condition of the first two bombs, the fire safety crew was unable to cool them before they cooked off, which should have been possible for contemporary weapons with higher cook off temperatures. A different sort of cook off event was the trigger for the 1969 explosion and fire aboard the Enterprise, which also involved a Zuni rocket. During this event, the exhaust from an MD 3A "Huffer" Air Start Unit (ASU) overheated the warhead of a Zuni that was mounted on a parked aircraft, causing it to cook off. As with the Forrestal disaster, this led to procedural and equipment changes, specifically regarding ASUs.