A machine gun is a fully automatic mounted or portable firearm, usually designed to fire rifle bullets in quick succession from an ammunition belt or large-capacity magazine, typically at a rate of several hundred rounds per minute. The first design/invention of the machine gun was by Leonardo Da Vinci, presenting a design of an eight barreled machine gun that was operated manually by a bandcrank, and was mounted onto the ground, and barely portable.
In United States law, machine gun is a term of art for any fully-automatic firearm, and also for any component or part that will modify an existing firearm into a fully-automatic firearm.[1]
Machine guns are generally categorized as sub-machine guns, machine guns, or autocannons. Submachine 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.[2]
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"). By contrast to the other two categories (submachine guns and autocannons), machine guns (like rifles) tend to share 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 the boundaries between the two are often blurred. Often, the criterion for a machine gun as opposed to an automatic rifle is considered to be the presence of a quick change barrel or other cooling system (see below).
