Machine Gun, a weapon loaded and fired automatically, designed to discharge a long-continued stream of projectiles of rifle caliber or slightly larger than rifle caliber. A machine gun is usually belt-fed, with cartridges attached to a belt and automatically fed into the breech. Most belts are of the disintegrating metal-link type in which the links making up the belt come apart upon firing. A few machine guns are magazinefed. (A magazine is a metallic container that brings one cartridge after another into loading position.)

A machine gun is similar to both an automatic rifle and an automatic cannon (used against aircraft). An automatic rifle, however, is lighter and, when firing in the automatic mode, less accurate; an automatic cannon fires a larger-diameter projectile, containing an explosive charge. The machine gun is one of the principal weapons of the infantry. It is a specialty weapon, meaning that relatively few troops in a unit are normally armed with it. The machine gun is also used on tanks and other military vehicles, helicopters, and slow-moving airplanes. The weapon once was commonly used as an antiaircraft gun and as armament for jet fighters and bombers, but it has been superseded for these purposes by automatic cannon.

Most machine guns are air-cooled, usually having aluminum fins or a perforated jacket to radiate heat from the barrel and set up currents of air to aid in cooling. Barrels quickly overheat in continuous fire and are therefore designed to be easily replaceable. Some machine guns, now largely obsolete, are water-cooled, having the barrel surrounded by a tubular jacket containing water.

Most machine guns are capable of firing at a rate of 450 to 700 rounds a minute; some have much higher rates. Because of the danger of overheating the weapon, the gunner does not actually fire this number of rounds within one minute. Machine guns are normally fired in short bursts.