Unlike semi-automatic firearms, which require one trigger pull per bullet fired, a machine gun is designed to fire as long as the trigger is held down and ammunition is fed into the weapon. Although the term "machine gun" is often used by civilians to describe all full automatic weapons, in military usage the term is restricted to relatively heavy weapons fired from some sort of support rather than hand-held, able to provide continuous or frequent bursts of automatic fire for as long as ammunition lasts. Machine guns are normally used against unprotected or lightly protected personnel, or to provide suppressive fire.
Some machine guns have in practice maintained suppressive fire almost continuously for hours; other automatic weapons overheat after less than a minute of use. Because they become very hot, practically all machine guns fire from an open bolt, to permit air cooling from the breech between bursts. They also have either a barrel cooling system, or removable barrels which allow a hot barrel to be replaced.
Although subdivided into "light", "medium", "heavy" or "general purpose", even the lightest machine guns tend to be substantially larger and heavier than other automatic weapons. Squad automatic weapons (SAW) are a variation of light machine gun and only require one operator (sometimes with an assistant to carry ammunition). Medium and heavy machine guns are either mounted on a tripod or on a vehicle; when carried on foot, the machine gun and associated equipment (tripod, ammunition, spare barrels) require additional crew members.
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.
Other automatic weapons are subdivided into several categories based on the size of the bullet used, and whether the cartridge is fired from a positively locked closed bolt, or a non-positively locked open bolt. Full automatic firearms using pistol-caliber ammunition are called machine pistols or submachine guns largely on the basis of size. Selective fire rifles firing a full-power rifle cartridge from a closed bolt are called automatic rifles or battle rifles, while rifles that fire an intermediate cartridge (see below) are called assault rifles. The difference in construction was driven by the difference in intended deployment. Automatic rifles (such as the Browning Automatic Rifle) were designed to be a high duty cycle arm for support of other troops, and were often made and deployed with quick change barrel assemblies to allow quick replacement of over heated barrels to allow for continued fire, and may have been operated by both the person actually firing the weapon as well as an additional crewman to assist in providing and caring for ammunition and the barrels, similar to a reduced version of a squad weapon (above). The assault rifle generally was made for a more intermittent duty cycle, and was designed to be easily carried and used by a single person.
Assault rifles are a compromise between the size and weight of pistol-caliber submachine gun and a full size traditional automatic rifle by firing intermediate cartridges, (or sometimes full power cartridges) and allowing semi-automatic, burst or full-automatic fire options (selective fire), often with two or more of these available on the rifle at once. The modern legal definition of "assault rifle" is of significance in states like California, where according to state law, certain weapons that resemble true assault rifles, but are only capable of semi-automatic (or autoloading), are categorized as "assault weapons" and are illegal to purchase or own by civilian residents of the state, even after a less restrictive ban by the federal government was allowed to lapse after having no impact on these weapons' use in crime. Therefore, supporters of gun rights generally consider the use of the phrase "assault weapon" to be pejorative when used to describe these civilian firearms, and this term is seldom used outside of the United States in this context.
The machine gun's primary role in modern ground combat is to provide suppressive fire on an opposing force's position, forcing the enemy to take cover and reducing the effectiveness of his fire . This either halts an enemy attack or allows friendly forces to attack enemy positions with less risk.
Light machine guns usually have simple iron sights. A common aiming system is to alternate solid ("ball") rounds and tracer ammunition rounds (usually one tracer round for every four ball rounds), so shooters can see the trajectory and "walk" the fire into the target, and direct the fire of other soldiers.
Many heavy machine guns, such as the Browning M2 .50 caliber machine gun, are accurate enough to engage targets at great distances. During the Vietnam War, Carlos Hathcock set the record for a long-distance shot at 7382 ft (2250 m) with a .50 caliber heavy machine gun he had equipped with a telescopic sight. This led to the introduction of .50 caliber anti-materiel sniper rifles, such as the Barrett M82.
