Machine Gun

I

Introduction

Machine Gun, type of heavy military weapon that fires ammunition continuously. Machine guns are known as automatic weapons because they automatically eject spent rounds of ammunition and reload fresh rounds. The user of a machine gun can fire a constant and rapid stream of ammunition merely by depressing and holding the trigger. (In contrast, a semiautomatic weapon also expels and reloads rounds, but the user must pull the trigger each time in order to fire a round.) Almost all machine guns use the energy released from the ammunition as it is fired to expel, reload, and fire ammunition. Some types of large machine guns, such as those used on fighter aircraft, are powered by motors.

Machine guns come in many sizes and fire small through intermediate-sized ammunition (5 to 40 mm in diameter). A machine gun can fire automatically for long periods of time but usually requires a way to cool its heavy barrel, which quickly heats up from firing. Early machine guns used air or water cooling, but these guns were often heavy or bulky. Many modern machine guns feature replaceable barrels, which can be changed when the barrel becomes overheated.

A common misconception is that the Russian AK-47 and the American M16 assault rifles are machine guns; they are more correctly called automatic rifles (see Small Arms). Both automatic rifles and machine guns can fire repeatedly when the user holds the trigger, but automatic rifles are much lighter than machine guns and are not designed for continuous firing. Also, automatic rifles use small clips of ammunition that hold only 20 or 30 rounds whereas machine guns use large magazines of up to 200 rounds or long belts of ammunition for continuous firing with minimum reloading.

II

How Machine Guns Work

Machine guns, as well as other automatic weapons, work by automatically expelling spent rounds of ammunition and reloading fresh rounds so that constant firing can be maintained. In order to accomplish automatic firing, a sequence of events takes place. First, a fresh round is inserted into the firing chamber, at the end of the gun barrel near the trigger. The breech, or gap, containing the round is sealed or locked in place. When the trigger is depressed, a pin strikes the back of the round, igniting the gunpowder in the round and propelling the bullet through the barrel. Once the round is fired, the breech is unlocked, the spent round is extracted from the firing chamber and ejected from the breech, and a fresh round is then inserted into the firing chamber.

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Self-powered machine guns use either blowback, recoil, or gas to move the mechanisms that reload, fire, and expel ammunition. Externally powered machine guns use motors to move these mechanisms.

A

Blowback

Blowback-operated weapons use the pressure created from the fired round to push a bolt, located directly behind the round, back and forth against a spring. The pressure from the fired round pushes the bolt backward against the spring and also ejects the spent round from the gun. A new round enters the weapon, and as the compressed spring pushes the bolt forward, the bolt rams the round into the breech. A pin on the end of the bolt strikes the round and fires it, beginning the cycle again. Blowback weapons are simple and reliable, but they do not form a complete seal at the breach when firing. This is a disadvantage, and blowback-operated weapons can be used only with ammunition that requires low to moderate firing pressure.

B

Recoil

Recoil-operated weapons push the barrel and the breech backwards as a unit, along with the bolt. The ejection and reloading cycles are completed during this recoil, and the breech remains sealed during firing. Recoil-operated weapons are extremely reliable, but are heavy and have more moving parts than blowback-operated weapons. As a result, they have relatively low rates of fire.

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