It is illegal to possess any of the following; a short barreled shotgun or riffle, silencer, or machine gun without registering them with the NFA. Exceptions are made to guns that have been modified so that they can not fire.
NFA weapons refer to a federal list of firearms that the government requires owners register. Not all states follow this law, and others have partial restrictions, full banns, or the need to register firearms in the NFA data base. Examples of firearms or other weaponry considered a NFA regulated weapon includes, but is not limited to: Machine guns, firearm suppressors, short barreled riffles and shot guns, and anti-tank guns (.50 caliber and up).