Title II weapons

A machine gun, as defined in the NFA, is "Any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot without manual reloading, by a single function of the trigger." The NFA term machine gun refers to all firearms capable of full automatic fire and includes true machine guns, submachine guns, and machine pistols. The frame or receiver of a machine gun, and any combination of parts intended to make a machine gun, is legally defined as a machine gun.[8] For example, according to the ATF, "A Glock conversion switch is a part designed and intended for use in converting a semi-automatic Glock pistol into a machine gun; therefore, it is a "machine gun" as defined in 26 U.S.C. 5845(b)." Open-bolt firearms made after 1982 are considered machine guns due to ease of conversion.[9]

Parts that can be used to convert a semi-automatic firearm to fully automatic capability are regulated as machine guns and must be registered and tax paid under the NFA. The U.S. military issued kits T17 and T18 to convert the M1 carbine to an M2, capable of fully automatic fire; these kits are legally "machine guns".[10]

A short-barreled shotgun (SBS) is defined as:[11]

(1) a shotgun having a smoothbore barrel or barrels of less than 18 inches in length;
(2) a weapon made from a shotgun if such weapon as modified has an overall length of less than 26 inches or a barrel or barrels of less than 18 inches in length [...]

It must be intended to be fired from the shoulder one shell of shot (pellets) or one projectile at a time.[12]

A short-barreled rifle (SBR) is defined as:[13]

(3) a rifle having a rifled barrel or barrels of less than 16 inches in length;
(4) a weapon made from a rifle if such weapon as modified has an overall length of less than 26 inches or a barrel or barrels of less than 16 inches in length [...]

A rifle is a firearm designed to be fired from the shoulder and fire one bullet at a time through a rifled barrel.[14] An SBR need not retain a shoulder stock after modification.

ATF regards pistols with shoulder stocks as redesigned to be fired from the shoulder. Modern pistols with shoulder stocks and with barrels less than 16 inches long, or overall length under 26 inches, are NFA short barrelled rifles. ATF has removed some specified stocked handguns (e.g., original Mauser C96 and Luger utilizing an original shoulder stock) from the NFA as collectors' items (Curios or Relics List);[15] ATF treats them as pistols under the GCA.[16]

The USAS-12 automatic shotgun is a "destructive device".

There are two categories of destructive devices (DDs):[17]

Explosive ordnance

Any explosive, incendiary, or poison gas, including bombs, grenades, rockets, missiles, mines and similar devices (e.g. grenade launchers, rocket launchers). Parts intended for making such a device are also DDs. Small rockets, with less than 4 ounces (113 grams) of propellant, are exempt.

Large bore firearms

Any projectile weapon with a bore diameter greater than ​¹⁄₂ inch (50 caliber, 12.7 mm), except for shotguns "generally recognized as particularly suitable for sporting purposes".

Most commercial shotguns have a bore diameter greater than ​¹⁄₂ inch, but are exempt due to their "sporting purpose"; however, both the Street Sweeper and USAS-12 shotguns, designed for military or police use, were reclassified as DDs when the ATF determined they were combat shotguns not "generally recognized as particularly suitable for sporting purposes".

Devices which are not intended or not likely to be used as a weapon are also exempt. Examples of non-weapon large bore firearms include:

• Line-throwing devices for marine rescue, such as lyle guns and rockets for breeches buoys.
• Civilian flare guns, which fire 37 mm flare caliber (1.46 inch) non-weapon rounds.

Fireworks are non-weapon explosive ordnance.

Flare launchers are normally exempt as they are signalling devices, not weapons; however, possession of a flare launcher and anti-personnel ammunition for it puts it in the DD category as it is then considered to be a weapon.

The legal term silencer, also known as a "suppressor", is defined as "any device for silencing, muffling, or diminishing the report of a portable firearm, including any combination of parts ... intended for use in assembling or fabricating a firearm silencer."[18]

The Serbu Super-Shorty pump-action shotgun, manufactured without a buttstock, is an AOW smooth-bore handgun, not an SBS.

A disguised firearm such as this cell phone gun is an AOW.

"Any other weapon" is a "catch-all" category. An AOW is defined as "any weapon or device capable of being concealed on the person from which a shot can be discharged through the energy of an explosive," other than a handgun with a rifled barrel".[3] This umbrella definition includes many improvised firearms ("zip guns") and disguised firearms. Examples include wallet guns, cane guns, knife guns and pen guns.[19] An AOW can be transferred to non-prohibited persons with a $5 BATF stamp as opposed to the $200 stamp required for machine guns and short-barreled rifles.

AOW is a complex and often misunderstood category of NFA firearms. Less obvious examples of AOW devices include:

Short-barreled shotguns manufactured without a shoulder stock (less than 26" overall length)

They are smooth-bore handguns which fire shot shells, not shotguns, which must be designed to be fired from the shoulder.

Pistols with a second vertical grip

Many pistols feature a rail below the barrel, commonly used to mount a laser or flashlight. Attaching a vertical grip to this rail constitutes the manufacturing of an AOW firearm, as it is "no longer designed to be held and fired by the use of a single hand."[20] It is therefore illegal to place an aftermarket vertical foregrip on any pistol without first registering it as an AOW and paying the $200 "making and registering tax". Failure to do so is a felony punishable by up to 10 years in prison.[20] However, if the receiver was originally manufactured to accept either a long or short barrel and a removable buttstock and fore grip and it can be assembled either as a rifle or a pistol, according to ATF rule 2011-4 it is not considered an NFA weapon as long as it is only assembled as a pistol without a buttstock or as a rifle with a barrel at least 16 inches long.[21] A vertical foregrip MAY be added to a pistol as long as the Overall Length (OAL) is greater than 26", regardless of barrel length, and providing the weapon remains unconcealed.[22] An AR-15 pistol with an overall length of 26" or longer may have a vertical foregrip installed, as long as no buttstock is installed in conjunction with a shorter than 16" barrel. The Sig Sauer Pistol Braces (SBS and SBX) are commonly found on these AR-15 style pistols with vertical foregrips since they are not considered buttstocks. The ATF does not consider a weapon in this configuration to be an AOW, but instead classifies it as a 'Firearm' which does not require any tax stamp or additional registration.

Firearms having combination rifle and shotgun barrels, more than 12 inches but less than 18 inches long from which only a single discharge can be made from either barrel without manual reloading

these are designed to be fired from the shoulder. An example is the Marble Game Getter an early 20th-century sporting gun sold before the NFA.[23]

Spud guns

These may potentially be classified as AOWs because they have a large bore and an unrifled barrel. One of the frequently asked questions on the BATF website FAQs is: "How do I obtain a classification from ATF for my "potato gun?"[24] It is not known at present if the BATF has actually classified any potato gun as an AOW. Such a classification would require the manufacturer to either pay the $200 manufacturing tax, surrender the weapon to the BATF, or destroy it. However, the vast majority of potato cannons, despite having a barrel diameter of much more than ½ inches, are built as muzzle-loading guns. Since these are not considered firearms (and thus completely unregulated) under US laws, building, owning, or selling potato cannons of traditional design is very unlikely to result in legal repercussions. However, any explosive rounds designed for these guns would be classified as destructive devices.

An antique firearm is excluded from NFA regulation if it is "not likely to be used as a weapon" and is not a machine gun or destructive device.[3] It must have been manufactured before 1899:

18 USC 921 (a)(16). (A) any firearm (including any firearm with a matchlock, flintlock, percussion cap, or similar type of ignition system) manufactured in or before 1898; and (B) any replica of any firearm described in subparagraph (A) if such replica -- (i) is not designed or redesigned for using rimfire or conventional centerfire fixed ammunition, or (ii) uses rimfire or conventional centerfire fixed ammunition which is no longer manufactured in the United States and which is not readily available in the ordinary channels of commercial trade.

Note that a firearm manufactured before 1899 is an antique by nature of production date. A replica is of the same status provided it is either a muzzle loader, uses non-fixed ammunition (such as pinfire), or custom ammunition not commercially produced (such as conversion cartridges). Note that ATF has not defined "commercial production" for quantities of ammunition to qualify as "readily available".

A few states, such as New York, Delaware and California, prohibit ownership of all or certain types of Title II weapons and devices. Most states allow legal ownership if the owner has complied with the federal registration and taxation requirements. A few states only allow possession of NFA firearms on the ATF Curios and Relics List, again only if the owner has complied with all federal requirements.