A machine gun is an auto-firing, rifled long-barrel autoloading firearm designed for sustained direct fire with fully powered cartridges. Other automatic firearms such as assault rifles and automatic rifles are typically designed more for firing short bursts rather than continuous firepower, and not considered machine guns. Squad automatic weapons, which fire the same (usually intermediate-powered) cartridge used by the other riflemen from the same combat unit, are functionally light machine guns though not called so. Submachine guns, which are capable of continuous rapid fire but using handgun cartridges, are also not technically regarded as true machine guns.

Similar automatic firearms of greater than 20 mm (0.79 in) caliber are classified as autocannons, rather than machine guns.

As a class of military kinetic projectile weapon, machine guns are designed to be mainly used as infantry support weapons and generally used when attached to a bipod or tripod, a fixed mount or a heavy weapons platform for stability against recoils. Many machine guns also use belt feeding and open bolt operation, features not normally found on other infantry repeating firearms.