Destructive devices, and silencers are illegal. Short barreled shotguns and machineguns are illegal, unless the are considered Curios and Relics.
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).