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. Insensitive munitions
are designed to be less vulnerable to accidental firing induced by external heat.
Inherent design flaws in early 17th century Swedish leather cannon
s 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 cook off if their guns were loaded when the vessels caught fire. Examples include the merchantman and .
After the cooking off of artillery shell
s in the G5 howitzer
s 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 gun
s 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. Nitrocellulose
, the primary component of modern smokeless powder
, has a relatively low autoignition temperature of around . 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.
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 gun
s (and submachine gun
s) 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.
Most modern infantry assault rifle
s 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.Colt Canada OPERATORS INSTRUCTIONS C7 FAMILY OF COMBAT WEAPONS stating at page 7; "The C7 family of weapons can fire the entire combat load of 150 rounds at any rate of fire without danger of cookoff."
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.
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 , 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 , 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.
* Sympathetic detonation