What are NFA Weapons?
There are two kinds of firearms under U.S. (federal) law, title 1 firearms and title 2. Title 1 firearms are long guns (rifles and shotguns), handguns, silencer, and firearm frames or receivers. Most NFA weapons are also title 1 firearms. Title 2 weapons are NFA weapons. Title 2 of the 1968 Gun Control Act is the National Firearms Act (codified at 26 U.S.C. sec. 5801 et seq.), hence NFA. Title 1 is generally called the Gun Control Act, (18 U.S.C. sec. 921 et seq.). NFA weapons are also sometimes called class 3 weapons, because a class 3 SOT is needed to deal in NFA weapons.
These weapons may also be further regulated by states or localities, and while these weapons can be legally owned under federal law, some states and localities further regulate ownership or prohibit it. The NFA Branch of ATF administers the taxation of the guns, and the registration of them in the National Firearms Registration and Transfer Record.
NFA weapons are: machine guns, sound suppressors (a.k.a. silencers), short barreled shotguns, short barreled rifles, destructive devices and "any other weapons". Exactly what these weapons are is defined in the law, as well as in court cases interpreting the law. Without going into to much detail, these are what the categories encompass:
A machine gun is any gun that can fire more than one shot with a single pull of the trigger, or a receiver of a machine gun, or a combination of parts for assembling a machine gun, or a part or set of parts for converting a gun into a machine gun.
A silencer is any device for muffling the gunshot of a portable firearm, or any part or parts exclusively designed or intended for such a device.
A short barreled shotgun is any shotgun (which is defined as a shoulder fired, smooth bore firearm) with a barrel of less than 18" or an overall length of less than 26", or any weapon made from a shotgun falling into the same length parameters.
A short barreled rifle is a rifle (which is defined as a shoulder fired, rifled bore firearm) with a barrel length of less than 16", or an overall length of less than 26", or any weapon made from a
rifle falling into the same length parameters (like a pistol made from a rifle). In measuring barrel length you do it from the closed breech to the muzzle, see 27 CFR sec. 179.11. To measure
overall length do so along, "the distance between the extreme ends of the weapon measured along a line parallel to the center line of the bore." 27 CFR sec. 179.11. On a folding stock weapon you measure with the stock extended, provided the stock is not readily
detachable, and the weapon is meant to be fired from the shoulder.
A destructive device (DD) can be two basic categories of things. It can be an explosive, incendiary or poison gas weapon, like a bomb or grenade. It can also be a firearm with a bore over 1/2", with exceptions for sporting shotguns, among other things.
I call the second category large bore destructive devices. As a general rule only this second category is commercially available.
Any other weapons (AOW's) are a number of things; smooth bore pistols, any pistol with more than one grip, gadget type guns (cane gun, pen gun) and shoulder fired weapons with both rifled and smooth bore barrels between 12" and 18", that must be manually reloaded.
These definitions are simplified, to see if a specific gun is a title 1 or 2 firearm one needs to refer to the specific definition under the statute(s), and possibly consult with the Technology Branch of ATF. There is also case law on the issue of whether a specific item falls into one of these categories. In addition, as a general rule, a parts kit, i.e. all of the parts to assemble an
NFA firearm, whether a parts kit is specifically included in the statute or not, is usually considered to be the same as the assembled firearm.