Under the NFA, even “firearm” has a specific legal meaning. This glossary explains the terms used in the chapters. The full definitions and exceptions are in the statutes linked below.
The law Congress passed in 1934 to tax and register specified firearms. It is now found mainly in Chapter 53 of the Internal Revenue Code. The NFA uses “firearm” for a narrower group of items than the word usually describes.
The broader federal gun law passed in 1968. It governs dealer licensing, interstate transactions, and who may possess firearms, among other matters. A firearm covered by the NFA can also be subject to the GCA.
Under federal law, a weapon that fires—or is designed or can readily be restored to fire—more than one shot automatically, without manual reloading, by a single function of the trigger. The definition also includes specified frames, receivers, and conversion components. Appearance alone does not make a semiautomatic firearm a machine gun.
A rifle with a barrel under 16 inches. The NFA also covers certain weapons made from rifles if the barrel is under 16 inches or the overall length is under 26 inches. Congress reduced the original 18-inch rifle-barrel threshold in 1960.
A shotgun with a barrel under 18 inches. The definition also includes certain weapons made from shotguns if the barrel is under 18 inches or the overall length is under 26 inches.
The statute uses “silencer”; “suppressor” is another common name for the same general category. The legal definition includes completed devices, specified combinations of parts, and parts intended for use in assembling a silencer.
A category for certain concealable weapons and other firearms specified in the statute. Despite its name, it has defined limits and exclusions; it is not a catchall for every weapon outside the other categories.
A category Congress added in 1968. It includes specified explosive devices and certain large-bore weapons, with exceptions set out in the law.
Short for National Firearms Registration and Transfer Record: the federal register of firearms covered by the NFA.
A tax paid by businesses carrying out specified NFA activities. It is separate from the taxes on making and transferring firearms that Congress changed in 2025.
A court order requiring someone to do something or to stop. The order identifies who is protected and what conduct it covers. Some injunctions apply only to the parties in a case.
An agency’s choice about how to enforce a law. A change in enforcement can affect owners and businesses even while the statute remains on the books.
Definitions are abbreviated. Read 26 U.S.C. §5845 and 18 U.S.C. §921 for the actual statutory language. For October 2026 enforcement policy, see Where the law stands.