A machine gun is a fully automatic mounted or portable firearm, usually designed to fire rounds in quick succession from an ammunition belt or large-capacity magazine, typically at a rate of several hundred rounds per minute.
Machine guns are generally categorized as sub-machine guns, machine guns, or autocannons. Sub-machine guns are designed to be portable automatic weapons for personal defense or short range combat, and are intended to be fired while being hand held. Submachine guns use small pistol caliber rounds. A proper machine gun is often portable to a certain degree, but is generally used when mounted on a stand or fired from the ground on a bipod. Light machine guns can be fired hand held like a rifle, but the gun is more effective when fired from a prone position. Proper machine guns use larger caliber rifle rounds. The difference between machine guns and autocannons is based on caliber, with autocannons using calibers larger than 16 mm.