A machine gun is a fully automatic mounted or portable firearm, usually designed to fire bullets in quick succession from an ammunition belt or magazine, typically at a rate of several hundred rounds per minute.
Machine guns are generally categorized as submachine guns, machine guns, or autocannons. Submachine guns are hand-held small portable automatic weapons for personal defense or short-range combat firing pistol-caliber rounds. A machine gun is often portable to a certain degree, but is generally used when attached to a mount or fired from the ground on a bipod, and generally fires a rifle cartridge. Light machine guns are small enough to be fired and are hand-held like a rifle, but are more effective when fired from a prone position. The difference between machine guns and autocannons is based on caliber, with autocannons using calibers larger than 16 mm.
Another factor is whether the gun fires conventional rounds or explosive rounds. Guns firing large-caliber explosive rounds are generally considered either autocannons or automatic grenade launchers ("grenade machine guns"). In contrast to submachine guns and autocannons, machine guns (like rifles) tend to share the characteristic of a very high ratio of barrel length to caliber (a long barrel for a small caliber); indeed, a true machine gun is essentially a fully automatic rifle, and often the primary criterion for a machine gun as opposed to an automatic rifle is the presence of a quick-change barrel or other cooling system (see below).
In United States gun law, machine gun is a technical term for any fully automatic firearm, and also for any component or part that will modify an existing firearm such that it functions as a fully automatic firearm.
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