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Operation
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 examplesare:
* 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 to their 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
These 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.
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.
