Cooking off

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

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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, 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 Earl Fiztwilliam and HMS Queen Charlotte

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.

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. 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 160–170 °C (320–338 °F). [1] 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:

Closed bolt

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. [2] 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 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.

Tanks

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 USS 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 USS Enterprise (CVN-65), 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.

See also

Related Research Articles

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A machine gun is a rapid-firing, rifled long-barrel autoloading firearm designed for sustained direct fire with full-powered cartridges. Other automatic firearms such as assault rifles and automatic rifles are really designed more for unleashing short bursts rather than continuous firepower, and not considered machine guns. Squad automatic weapons, which fire the same cartridge used by the other riflemen from the same combat unit, are functionally light machine guns though not called so. Submachine guns, which are capable of continuous rapid fire but using handgun cartridges, are also not technically regarded as true machine guns.

Cartridge (firearms)

A cartridge or a round is a type of pre-assembled firearm ammunition packaging a projectile, a propellant substance and an ignition device (primer) within a metallic, paper or plastic case that is precisely made to fit within the barrel chamber of a breechloading gun, for the practical purpose of convenient transportation and handling during shooting. Although in popular usage the term "bullet" is often used to refer to a complete cartridge, it is correctly used only to refer to the projectile.

MK 108 cannon Type of Autocannon

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Heckler & Koch G11 Caseless ammunition assault rifle prototype

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Caseless ammunition

Caseless ammunition is a type of small arms ammunition that eliminates the cartridge case that typically holds the primer, propellant, and projectile together as a unit.

Gun barrel Firearm component which guides the projectile during acceleration

A gun barrel is a crucial part of gun-type ranged weapons such as small firearms, artillery pieces and air guns. It is the straight shooting tube, usually made of rigid high-strength metal, through which a contained rapid expansion of high-pressure gas(es) is used to propel a projectile out of the front end (muzzle) at a high velocity. The hollow interior of the barrel is called the bore, and the diameter of the bore is called its caliber, usually measured in inches or millimetres.

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MG 15 Type of Machine gun

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Open bolt

A fully automatic firearm is said to fire from an open bolt 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 cartridge from the magazine or belt into the chamber, and fires that cartridge in the same movement. Like any other self-loading design, the action is cycled by the energy released from the propellant, which sends the bolt back to the rear, compressing the mainspring in readiness for firing the next round. In an open-bolt gun firing semi-automatically, the bolt is caught and held at this point by the sear after each shot; and in automatic open-bolt fire, it's caught and held in this manner whenever the trigger is released. In contrast to this, in closed-bolt guns the trigger and sear don't affect the movement of the bolt directly.

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Gas-operated reloading

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Slamfire

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The following are terms related to firearms and ammunition topics.

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