Machine gun
A .50 caliber M2 machine gun surrounded by spent shell casings: John Browning''s design has been one of
the longest serving and successful machine gun designs
An illustration of later model 19th century Gatling gun. It was a multi-barreled rotary machine gun fired via a manual crank, which mechanically loaded and primed cartridges from the hopper above the gun
A modern image of an 1895 tripod-mounted, .303 caliber Maxim machine gun. The original Maxim of the 1880s was the first fully automatic machine gun, as well as using a a belt of linked ammunition rather than a hopper
7.62 mm machine gun on top of a Leclerc tank
For other uses of the phrase, see Machine gun (disambiguation). A
machine gun is a fully-automatic mounted or portable firearm, usually designed to fire rifle cartridges in quick succession from an ammunition belt or large-capacity magazine, typically at a rate of several hundred bullets per minute. Earlier machine guns were manually
operated, for example, by turning a hand crank.
In United States law,
machine gun is a term of art for any fully-automatic firearm.<1>
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Components
All machine guns require the following components:
A feed system to load the chamber. Cartridges can be fed into the chamber by a variety of methods, the most common being magazines or ammunition belts.
A trigger
mechanism to fire the round. This includes the actual trigger, a trigger sear to catch the bolt, a bolt and a firing pin, as well as other components. Typically, the act of pulling the trigger causes something to strike the primer on the round in the chamber and disengages the sears. This allows continual cycling of the bolt until the trigger is released. A sear then grabs the bolt or firing pins. This stops the machine gun at some point in its cycle.
An extractor system to eject the spent or misfired cartridge. Usually this is fairly simple. A pin on the side of the bolt catches a ridge on the cartridge and flicks it out an ejection port.
These components form a mechanism which must be powered. If powered by absorbing the recoil of a cartridge, it is called recoil-operated. If powered by the expanding gases of a fired cartridge, it is called gas actuated. If powered by an external force, such as a motor, it is usually called a chain gun.
Operation
An M60 machine gun aboard a Navy patrol craft. The USS Constellation (CV-64) in the distance; July 2002
All machine guns follow a cycle:
Removing the spent cartridge through an ejection port.
Cocking the trigger mechanism so the weapon can be fired again.
Loading the next round into the firing chamber. Usually spring tension or a cam forces the new round and bolt back into the firing chamber.
A mechanism makes the firing pin fire the cartridge, activating the ejection and reloading steps. The cycle repeats. This full cycle takes a fraction of a second and can thus occur many times per second. The operation is basically the same, regardless of the means of activating these mechanisms. Some examples:
Machine pistols and submachine guns (like the World War II "grease gun," MAC-10 or the Uzi) are usually blowback operated.
Most assault rifles and squad automatic weapons are gas actuated. Some weapons, such as the AR-15/M16, integrate the piston with the bolt. Others, such as the AR18 and AK patterns, attach the piston to a bolt carrier that unlocks and operates the bolt.
A recoil-actuated machine gun uses the recoil to first unlock and then operate the action. Heavy machine guns, such as the M2 .50 and Browning .50, are of this type. These can be recognized by a large cocking lever needed to feed the first round.
An externally actuated machine gun uses an external power source, such as an electric motor or even a hand crank to move its mechanism through the firing sequence. Most modern weapons of this type are called chain guns in reference toheir driving mechanism. Gatling guns and revolver cannon have several barrels or chambers on a rotating carousel and a system of cams that load, cock, and fire each mechanism progressively as it rotates through the sequence. The continuous nature of the rotary action allows for an incredibly high cyclic rate of fire, often several thousand rounds per minute. Not all chain guns use multiple barrels or chambers, though. Chain 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. This is not possible if the force needed to eject the round comes from the round itself. Chain guns are generally used with large shells, 20 mm in diameter or more, though some, such as the M134 minigun, fire smaller cartridges. They offer benefits of reliability and firepower, though the weight and size of the power source and driving mechanism makes them impractical for use outside of a vehicle or aircraft mount.
Heavy machine guns are often water cooled or have interchangeable barrels, which must be changed periodically to avoid overheating. 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.
Not all machine guns strike the primer in the same way. In blowback machine guns, the act of seating the round also fires the round. In gas operated and recoil-operated guns, a separate step in the firing sequence is needed to strike the round. In a progressive-fire gun, the firing pin is cycled by cams. In some automatic cannon, the primer is fired electrically.
U.S. Marines and their M240G at Camp Hansen, Okinawa
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. This is especially important in weapons like the 40 mm grenade launcher, where high explosives are present in the rounds being fired.
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 weapons have a "safety" sear, which simply keeps the trigger from engaging.