The National Firearms Act of 1934 (NFA) defines a number of categories of regulated firearms. These weapons are collectively known as NFA firearms and include the following:
Machine guns—this includes any firearm which can fire more than 1 cartridge per trigger pull. Both continuous fully automatic fire and "
burst fire" (e.g., firearms with a 3-round burst feature) are considered machine gun features. The weapon's receiver is by itself considered to be a regulated firearm.
A non-machinegun that may be converted to fire more than one shot per trigger pull by ordinary mechanical skills is determined to be "readily convertible", and classed as a machinegun, such as a
KG-9 pistol (pre-ban ones are "grandfathered").