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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.