A machine gun is a fully automatic mounted or portable firearm, usually designed to fire rifle bullets in quick succession from an ammunition belt or large-capacity magazine, typically at a rate of several hundred rounds per minute. Machine guns are known as automatic weapons because they automatically eject spent rounds of ammunition and reload fresh rounds. Unlike semi-automatic firearms, which require one trigger pull per bullet fired, a machine gun is designed to fire bullets as long as the trigger is held down and ammunition is fed into the weapon. Machine guns are generally categorized as submachine guns, machine guns, or autocannons.
The majority of machine guns are belt-fed, although some light machine guns are fed from drum or box magazines, and some vehicle-mounted machine guns are hopper-fed. 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.
The distinction between submachine guns and machine guns is subtle, hinging upon whether the ammunition used is intended for use in pistols (chiefly semi-automatic pistols) or rifles; the difference between machine guns and autocannons is based on caliber, with autocannons using calibers larger than 16 mm.
Another factor is whether the gun fires conventional rounds or explosive rounds. Guns firing large-caliber explosive rounds are generally considered either autocannons or automatic grenade launchers.
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.
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.
Machine guns are grouped into four general categories: light machine guns; general-purpose, or medium, machine guns; heavy machine guns; and externally powered machine guns.
Light Machine Guns
Light machine guns use ammunition of .30 caliber (7.62 mm) or less and are designed to have minimum weight so that a single soldier may use them. The United States M249 Squad Automatic Weapon (SAW) is a portable, lightweight machine gun that fires 5.56-mm ammunition from continuous linked belts, or from linked ammunition contained in large-capacity magazines. The SAW is light enough to be operated by one soldier, but it can fire for longer periods of time than an automatic rifle. The SAW replaced the older U.S. M60 machine gun, which weighed nearly twice as much as the SAW.
Medium Machine Guns
General-purpose, or medium machine guns ordinarily use 7.62-mm ammunition. They are used for firing at more distant targets than light machine guns and can fire for longer periods of time. Medium machine guns are also heavier, requiring teams of at least two soldiers to use them. A medium machine gun requires a tripod (three-legged stand) for support, or a mounting bracket if used on a vehicle. The M60 served as a general-purpose machine gun for many years in the U.S. Army. It could be fired from a bipod (two-legged stand), from a tripod, from a vehicle, or from the side of a helicopter. Beginning in the 1980s, the M60 was gradually replaced by newer, more improved machine guns such as the M240.
Heavy Machine Guns
Heavy machine guns fire .50-caliber (12.7-mm) or larger ammunition and generally require a crew. Heavy machine guns can deliver concentrated and devastating firepower. They often serve as the main armament on land combat vehicles and on some aircraft.
Two types of heavy machine guns are the M2 machine gun, esigned by John Moses Browning, and the Mark 19 machine gun, which fires 40mm grenades that explode upon impact, making the weapon useful against both armored targets and enemy soldiers.
Externally Powered Machine Guns
Externally powered weapons are found in many military aircraft, in some vehicles, and in anti-aircraft units on the ground or aboard naval vessels. Many externally powered machine guns are modern motorized versions of the hand-turned Gatling gun. Gatling guns feature several barrels that rotate around a central shaft. Gatling guns can achieve high rates of fire without overheating. The 30-mm Gatling gun on the A-10 Thunderbolt II attack aircraft can fire 3,900 rounds a minute and is capable of destroying armored targets such as tanks.