Machineguns

A machine gun is a fully automatic mounted or portable firearm designed to fire bullets in quick succession from an ammunition belt or magazine, typically at a rate of 300 to 1800 rounds per minute. Note that not all fully automatic firearms are machine guns. Submachine guns, rifles, assault rifles, shotguns, pistols or may be capable of fully automatic fire, but are not designed for sustained fire. As a class of military rapid-fire guns, true machine guns are fully automatic weapons designed to be used as support weapons and generally used when attached to a mount or fired from the ground on a bipod or tripod. Many (though by no means all) machine guns also use belt feeding and open bolt operation, features not normally found on rifles.

In United States gun law, "machine gun" is also a legal term for any weapon able to fire more than one shot per trigger pull regardless of caliber, the receiver of any such weapon, any weapon convertible to such a state using normal tools, or any component or part that will modify an existing firearm such that it functions as a "machine gun" such as a drop-in auto sear. Civilian possession of such weapons is not prohibited by any Federal law and not illegal in many states, but they must be registered as Title II weapons under the National Firearms Act and have a tax stamp paid. The Hughes Amendment to the Firearm Owners Protection Act of 1986 banned new production of firearms classified as machine guns for most civilian applications, however, so only "grandfathered" weapons produced before this date are legally transferable.

...
Wikipedia