Enacted on June 26, 1934, The National Firearms Act (NFA) is an Act of Congress in the United States that, in general, imposes a statutory excise tax on the manufacture and transfer of certain firearms. The NFA also mandates the registration of those firearms.
There are two kinds of firearms under U.S. (federal) law, Title 1 and Title 2.
Title 1 firearms are long guns (rifles and shotguns), handguns and firearm frames or receivers. Title 1 is generally called the Gun Control Act (18 U.S.C. sec. 921 et seq.). These are guns you can purchase by filling out a 4473 Form and can be purchased over the counter here in TN.
Title 2 firearms are NFA firearms. 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. Most NFA firearms are also Title 1 firearms. Also, NFA firearms are sometimes called Class 3 firearms, because a Class 3 SOT is needed to deal in NFA firearms.
These firearms may be further regulated by states or localities, and while these firearms can be legally owned under federal law, some states and localities regulate ownership or even prohibit it. The NFA Branch of ATF administers taxation of the guns and the registration of them in the National Firearms Registration and Transfer Record.
NFA firearms are: machine guns, sound suppressors (a.k.a. silencers), short barreled shotguns, short barreled rifles, destructive devices and "any other firearms". Exactly what these firearms are is defined in the law, as well as in court cases interpreting the law.
In general, 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.
SHORT BARRELED RIFLE
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 firearm 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 firearm measured along a line parallel to the center line of the bore." 27 CFR sec. 179.11. On a folding stock firearm you measure with the stock extended, provided the stock is not readily detachable, and the firearm is meant to be fired from the shoulder.
SHORT BARRELED SHOTGUN
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 firearm made from a shotgun falling into the same length parameters.
DESCTRUCTIVE DEVICE (DD)
A destructive device (DD) can be two basic categories of things. It can be an explosive, incendiary or poison gas firearm, 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 firearm (AOW)
Any other firearms (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 firearms 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 Title 2 firearm, one needs to refer to the specific definition under the statute(s) and possibly consult with the Technology Branch of ATF. There's also case law on the issue of whether a specific item falls into one of these categories.
Also, as a general rule, a parts kit (all 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.
The National Firearms Act of 1934 makes it illegal for civilians to own machine guns without permission from the Federal Government. The National Firearms Act of 1934 levies a $200 tax on each newly manufactured machine gun and a $200 tax each time the ownership of the machine gun changes.
To purchase an NFA firearm, you must submit two sets of fingerprints, a recent photo, a sworn affidavit that transfer of the NFA firearm is of "reasonable necessity" and that sale-to and possession-of the firearm by the applicant "would be consistent with public safety" and endure a background investigation.
Law enforcement, states, and local governments are totally exempt from the making and transfer (either to or from) taxes, but must comply with the registration requirements. While the NFA only specifically provides that there is no transfer tax due when the U.S. government is the transferee, (26 U.S.C. sec. 5852(a)), or a state governmental entity (26 U.S.C. sec. 5853(a)), ATF has made up an exemption from the transfer tax where any U.S. or state governmental entity is the transferor, see ATF Chief Counsel Opinion numbers 20023 and 20400.
Abuses of this tax exemption, as in transferring guns through governmental entities so as to avoid transfer taxes, have been successfully prosecuted. See U.S. v. Fleming, 19 F.3d 1325 (CA10 1994). Federal government agencies, the military, and National Guard are exempt from the registration or tax requirements and generally speaking, NFA Branch removes firearms from transferrable status in the Registry once they are transferred to the federal government.