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. Although the term "machine gun" is often
used by civilians to describe all fully 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
Some machine guns have in practice maintained suppressive fire almost continuously for hours; other automatic weapons
overheat after sometimes 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
(SAWs) are a variation of light machine guns 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.