Editing Machine gun (section)

== Design ==
[[File:Lewis gun reloading mechanism.gif|thumb|310px|[[Lewis gun]] reloading mechanism action]]
Most modern machine guns are of the locking type, and of these, most utilize the principle of [[gas-operated reloading]], which taps off some of the propellant gas from the fired cartridge, using its mechanical pressure to unlock the bolt and cycle the action.  The first of these was invented by the French brothers Claire, who patented a gas operated rifle, which included a gas cylinder, in 1892. The Russian [[PK machine gun]] is a more modern example. Another efficient and widely used format is the [[Recoil operation|recoil actuated]] type, which uses the gun's recoil energy for the same purpose. Machine guns, such as the [[M2 Browning machine gun|M2 Browning]] and [[MG42]], are of this second kind. A cam, lever or actuator absorbs part of the energy of the recoil to operate the gun mechanism.

An externally actuated weapon uses an external power source, such as an electric motor or hand crank, to move its mechanism through the firing sequence. Modern weapons of this type are often referred to as [[Gatling gun]]s, after the original inventor (not only of the well-known hand-cranked 19th century proto-machine gun, but also of the first electrically powered version). They have several barrels each with an associated chamber and action on a rotating carousel and a system of cams that load, cock, and fire each mechanism progressively as it rotates through the sequence; essentially each barrel is a separate bolt-action rifle using a common feed source. The continuous nature of the rotary action and its relative immunity to overheating allow for a very high cyclic [[rate of fire]], often several thousand rounds per minute. Rotary guns are less prone to jamming than a gun operated by gas or recoil, as the external power source will eject misfired rounds with no further trouble; but this is not possible in the rare cases of self-powered rotary guns. Rotary designs are intrinsically comparatively bulky and expensive and are therefore generally used with large rounds, 20&nbsp;mm in diameter or more, often referred to as [[rotary cannon]] – though the rifle-calibre [[Minigun]] is an exception to this. Whereas such weapons are highly reliable and formidably effective, one drawback is that the weight and size of the power source and driving mechanism makes them usually impractical for use outside of a vehicle or aircraft mount.

[[Revolver cannon]]s, such as the [[Mauser MK 213]], were developed in World War II by the Germans to provide high-caliber cannons with a reasonable rate of fire and reliability. In contrast to the rotary format, such weapons have a single barrel and a recoil-operated carriage holding a revolving chamber with typically five chambers. As each round is fired, electrically, the carriage moves back rotating the chamber which also ejects the spent case, indexes the next live round to be fired with the barrel and loads the next round into the chamber. The action is very similar to that of the revolver pistols common in the 19th and 20th centuries, giving this type of weapon its name. A [[chain gun]] is a specific, patented type of revolver cannon, the name, in this case, deriving from its driving mechanism.

[[File:Machine gun feeding mechanism CC BY-SA 4.0 by Grasyl.svg|thumb|Machine gun belt feeding mechanism]]As noted above, firing a machine gun for prolonged periods produces large amounts of heat. In a worst-case scenario, this may cause a cartridge to overheat and detonate even when the trigger is not pulled, potentially leading to damage or causing the gun to cycle its action and keep firing until it has exhausted its ammunition supply or jammed; this is known as ''[[cooking off]]'' (as distinct from ''runaway fire'' where the [[sear (firearm)|sear]] fails to re-engage when the trigger is released). To guard against cook-offs occurring, some kind of cooling system or design element is required. Early machine guns were often [[Water cooling|water-cooled]] and while this technology was very effective (and was indeed one of the sources of the notorious efficiency of machine guns during World War I), the water jackets also added considerable weight to an already bulky design; they were also vulnerable to the enemies' bullets themselves. Armour could be provided, and in World War I, the Germans in particular often did this; but this added yet more weight to the guns. Air-cooled machine guns often feature quick-change barrels (often carried by a crew member), passive cooling fins, or in some designs forced-air cooling, such as that employed by the [[Lewis Gun]]. Advances in metallurgy and the use of special composites in barrel liners have allowed for greater heat absorption and dissipation during firing. The higher the rate of fire, the more often barrels must be changed and allowed to cool. To minimize this, most air-cooled guns are fired only in short bursts or at a reduced rate of fire. Some designs – such as the many variants of the [[MG42]] – are capable of rates of fire in excess of 1,200 rounds per minute. Motorized Gatling guns can achieve the fastest firing rates of all, partly because this format involves extra energy being injected into the system from outside, instead of depending on energy derived from the propellant contained within the cartridges, partly because the next round can be inserted simultaneously with or before the ejection of the previous cartridge case, and partly because this design intrinsically deals with the unwanted heat very efficiently – effectively quick-changing the barrel and chamber after every shot.  The multiple guns that comprise a Gatling being a much larger bulk of metal than other, single-barreled guns, they are thus much slower to rise in temperature for a given amount of heat, while at the same time they are also much better at shedding the excess, as the extra barrels provide a larger surface area from which to dissipate the unwanted thermal energy.  In addition to that, they are in the nature of the design spun at very high speed during rapid fire, which has the benefit of producing enhanced air-cooling as a side-effect.

In weapons where the round seats and fires at the same time, mechanical timing is essential for operator safety, to prevent the round from firing before it is seated properly. Machine guns are controlled by one or more mechanical sears. When a sear is in place, it effectively stops the bolt at some point in its range of motion. Some sears stop the bolt when it is locked to the rear. Other sears stop the firing pin from going forward after the round is locked into the chamber. Almost all machine guns have a "safety" sear,{{citation needed|date=December 2013}} which simply keeps the trigger from engaging.