Editing Cooking off (section)

== Machine guns ==
Cooking off is a characteristic of certain air-cooled [[machine gun]]s firing from a closed [[bolt (firearm)|bolt]]. In such a design, when the [[trigger (firearms)|trigger]] is released the weapon feed leaves a final round in the [[chamber (firearms)|chamber]]. Residual heat [[conduction (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 {{convert|160|-|170|°C|°F}}.<ref>{{cite web |url=https://pubchem.ncbi.nlm.nih.gov/compound/44263835#section=Melting-Point |title=Chemical and Physical Properties of Nitrocellulose}}</ref> 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.
* [[Cooling]]: [[gun barrel|Barrels]] can be [[coolant|liquid-cooled]] (like a [[radiator (engine cooling)|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.

=== Closed bolt ===
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-loading weapon|breech]]. Squeezing the trigger releases the [[firing pin|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.<ref>{{cite web |url=http://www.coltcanada.com/techpubs.htm |work=Colt Canada |title=OPERATORS INSTRUCTIONS C7 FAMILY OF COMBAT WEAPONS |page=7 |quote=The C7 family of weapons can fire the entire combat load of 150 rounds at any rate of fire without danger of cookoff. |archive-url=https://web.archive.org/web/20110708175626/http://www.coltcanada.com/techpubs.htm |archive-date=July 8, 2011 |url-status=dead }}</ref> 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 (firearms)|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.