Cooking off (or thermally induced firing) is unfired weapon

ammunition Ammunition, also known as ammo, is the material fired, scattered, dropped, or detonated from any weapon or weapon system. The term includes both expendable weapons (e.g., bombs, missiles, grenades, land mines), and the component parts of oth ...

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.

Artillery

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

gunpowder Gunpowder, also commonly known as black powder to distinguish it from modern smokeless powder, is the earliest known chemical explosive. It consists of a mixture of sulfur, charcoal (which is mostly carbon), and potassium nitrate, potassium ni ...

, injuring the loader. Muzzle-loading cannon on merchant and naval vessels of the

Age of Sail The Age of Sail is a period in European history that lasted at the latest from the mid-16th (or mid-15th) to the mid-19th centuries, in which the dominance of sailing ships in global trade and warfare culminated, particularly marked by the int ...

would fire if the vessels caught fire while the guns were loaded. Examples include the merchantman and . After the cooking off of

artillery shell A shell, in a modern military context, is a projectile whose payload contains an explosive, incendiary device, incendiary, or other chemical filling. Originally it was called a bombshell, but "shell" has come to be unambiguous in a military ...

s in the

G5 howitzer The G5 is a South African towed howitzer of 155 mm calibre developed in South Africa by Denel Land Systems. The G5 design was based on the Canadian GC-45 howitzer, GC-45 155 mm gun which was highly modified to suit southern African conditio ...

s in the late 1980s, the

South African Army The South African Army is the principal Army, land warfare force of South Africa, a part of the South African National Defence Force (SANDF), along with the South African Air Force, South African Navy and South African Military Health Servi ...

changed commands from "cease fire" to "cease loading" . This allowed crews to fire any loaded shells to prevent them from heating up and exploding.

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 A cartridge, also known as a round, is a type of pre-assembled firearm ammunition packaging a projectile (bullet, shot, or slug), a propellant substance ( smokeless powder, black powder substitute, or black powder) and an ignition device ( pr ...

. If the kindling point of the

propellant A propellant (or propellent) is a mass that is expelled or expanded in such a way as to create a thrust or another motive force in accordance with Newton's third law of motion, and "propel" a vehicle, projectile, or fluid payload. In vehicle ...

is eventually reached it will burn even though the primer has not been struck, thus firing the chambered round.

Nitrocellulose Nitrocellulose (also known as cellulose nitrate, flash paper, flash cotton, guncotton, pyroxylin and flash string, depending on form) is a highly flammable compound formed by nitrating cellulose through exposure to a mixture of nitric acid and ...

, the primary component of modern

smokeless powder Finnish smokeless powder Smokeless powder is a type of propellant used in firearms and artillery that produces less smoke and less fouling when fired compared to black powder. Because of their similar use, both the original black powder formula ...

, 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 A slamfire is a discharge of a firearm occurring as soon as a Cartridge (firearms), cartridge is being loaded into the Chamber (weaponry), chamber. Some firearms are designed to slamfire, but the term also describes a malfunction of self-loadin ...

) 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

: Among its many functions, the metallic

acts as a

heat sink A heat sink (also commonly spelled heatsink) is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant, where it is thermal management (electronics), ...

protecting the

from chamber heat. The case must first be brought up to temperature before the propellant inside can burn. *

Cooling Cooling is removal of heat, usually resulting in a lower temperature and/or Phase transition, phase change. Temperature lowering achieved by any other means may also be called cooling. The Heat transfer, transfer of Internal energy, thermal energ ...

: Barrels can be liquid-cooled (like a

radiator A radiator is a heat exchanger used to transfer thermal energy from one medium to another for the purpose of cooling and heating. The majority of radiators are constructed to function in cars, buildings, and electronics. A radiator is always a ...

in an

automobile engine There are a wide variety of propulsion systems available or potentially available for automobiles and other vehicles. Options included internal combustion engines fueled by petrol, diesel, propane, or natural gas; hybrid vehicles, plug-in hybr ...

), or exchanged periodically. Most modern infantry machine guns (GPMG,

general-purpose machine gun A general-purpose machine gun (GPMG) is an air-cooled, usually belt-fed machine gun that can be adapted flexibly to various tactical roles for light and medium machine guns. A GPMG typically features a quick-change barrel design calibered fo ...

) 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 A firearm is said to fire from an open bolt or open breech if, when ready to fire, the bolt and working parts are held to the rear of the receiver, with no round in the chamber. When the trigger is actuated, the bolt travels forward, feeds a car ...

: Most modern

infantry Infantry, or infantryman are a type of soldier who specialize in ground combat, typically fighting dismounted. Historically the term was used to describe foot soldiers, i.e. those who march and fight on foot. In modern usage, the term broadl ...

machine guns (and

submachine gun A submachine gun (SMG) is a magazine (firearms), magazine-fed automatic firearm, automatic carbine designed to fire handgun cartridges. The term "submachine gun" was coined by John T. Thompson, the inventor of the Thompson submachine gun, to descri ...

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.

Closed bolt

Most modern infantry

assault rifle An assault rifle is a select fire rifle that uses an intermediate cartridge, intermediate-rifle cartridge and a Magazine (firearms), detachable magazine.C. Taylor, ''The Fighting Rifle: A Complete Study of the Rifle in Combat'', F.A. Moyer '' ...

s fire from a

closed bolt A semi or full-automatic firearm which is said to fire from a closed bolt or closed breech is one where, when ready to fire, a round is in the chamber and the bolt and working parts are forward ''in battery''. When the trigger is pulled, the ...

, 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

Caseless ammunition Caseless ammunition (CL), or caseless cartridge, is a configuration of Cartridge (firearms), weapon-cartridge that eliminates the cartridge case that typically holds the Percussion cap, primer, propellant and projectile together as a unit. Instea ...

eliminates the

metal A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...

case that typically holds the primer or igniter and the powder charge (

) that propels the

bullet A bullet is a kinetic projectile, a component of firearm ammunition that is shot from a gun barrel. They are made of a variety of materials, such as copper, lead, steel, polymer, rubber and even wax; and are made in various shapes and constru ...

. The metal case absorbs a large portion of the

waste heat Waste heat is heat that is produced by a machine, or other process that uses energy, as a byproduct of doing work. All such processes give off some waste heat as a fundamental result of the laws of thermodynamics. Waste heat has lower utility ...

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.

Tanks

Cooking off is a serious hazard to crews in damaged and disabled

tank A tank is an armoured fighting vehicle intended as a primary offensive weapon in front-line ground combat. Tank designs are a balance of heavy firepower, strong armour, and battlefield mobility provided by tracks and a powerful engine; ...

s. Attempted solutions include storing ammunition under water and insulating ammunition compartments. The current technique, used in tanks such as the

M1 Abrams The M1 Abrams () is a third-generation American main battle tank designed by Chrysler Defense (now General Dynamics Land Systems) and named for General Creighton Abrams. Conceived for modern armored ground warfare, it is one of the heavies ...

, is to

armor Armour (Commonwealth English) or armor (American English; see American and British English spelling differences#-our, -or, spelling differences) is a covering used to protect an object, individual, or vehicle from physical injury or damage, e ...

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 carrier An aircraft carrier is a warship that serves as a seagoing airbase, equipped with a full-length flight deck and hangar facilities for supporting, arming, deploying and recovering carrier-based aircraft, shipborne aircraft. Typically it is the ...

s. 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 tank A fuel tank (also called a petrol tank or gas tank) is a safe container for Flammability, flammable fluids, often gasoline or diesel fuel. Though any storage tank for fuel may be so called, the term is typically applied to part of an engine sys ...

s of a waiting A-4 Skyhawk) detonated two

unguided bomb An unguided bomb, also known as a free-fall bomb, gravity bomb, dumb bomb, or iron bomb, is an aircraft-dropped bomb (conventional or nuclear) that does not contain a guidance system and hence simply follows a ballistic trajectory. It includes ...

s of

Korean War The Korean War (25 June 1950 – 27 July 1953) was an armed conflict on the Korean Peninsula fought between North Korea (Democratic People's Republic of Korea; DPRK) and South Korea (Republic of Korea; ROK) and their allies. North Korea was s ...

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.

References

{{Firearms Ammunition Military terminology