Under federal law, an “other firearm” is one of two very different things, and mixing them up is how people end up in federal court. The first is an “Any Other Weapon” (AOW) under the National Firearms Act: a concealable, disguised, or unusually configured weapon that must be registered with the ATF before you can own it. The second is a firearm that simply doesn’t meet the legal definition of a rifle, shotgun, or handgun under the Gun Control Act, but isn’t small enough to trigger NFA rules either. The Mossberg 590 Shockwave is the best-known example of the second type. On ATF Form 4473, the background check form used at every gun store, both types get checked as “Other Firearm” rather than “Handgun” or “Long Gun.”1Bureau of Alcohol, Tobacco, Firearms and Explosives. Firearms Transaction Record – ATF Form 4473
The two share a label. They share almost nothing else legally.
The AOW Category Under the NFA
An “Any Other Weapon” is defined by statute as any weapon or device capable of being concealed on a person that can fire a shot using an explosive charge. The definition specifically excludes pistols and revolvers with rifled bores and weapons designed to be fired from the shoulder. It also expressly includes smooth-bore pistols designed to fire shotgun shells and combination rifle-and-shotgun weapons with barrels between 12 and 18 inches that fire a single shot from either barrel without reloading.2Office of the Law Revision Counsel. 26 USC 5845 – Definitions
In practice, the AOW category catches disguised firearms like pen guns, cane guns, and wallet holsters that let a firearm be discharged while still inside the wallet. It also catches configurations that most owners wouldn’t expect. A pistol-grip-only shotgun with an overall length under 26 inches is an AOW because at that size it’s concealable. Adding a vertical foregrip to a handgun creates an AOW, because the weapon is no longer designed to be fired with one hand but is still small enough to conceal.3Bureau of Alcohol, Tobacco, Firearms and Explosives. Adding a Vertical Fore Grip to a Handgun
Every AOW must be registered in the National Firearms Registration and Transfer Record before you possess or make it.4Office of the Law Revision Counsel. 26 USC 5841 – Registration of Firearms Possessing an unregistered one is a federal felony.
The Non-NFA “Other Firearm”
The commercially popular version of an “other firearm” is a weapon that threads a narrow gap in the law: it isn’t a rifle, shotgun, or handgun, but it also isn’t small enough to be an AOW. The Mossberg 590 Shockwave and the discontinued Remington Tac-14 are the flagship examples. Both have 14-inch smooth-bore barrels and bird’s head grips rather than shoulder stocks.
The ATF’s reasoning on the Shockwave walks through each category and rejects all of them. The smooth bore rules out “rifle.” The absence of any shoulder stock in its original design rules out “shotgun,” because a shotgun is statutorily one that is “designed to be fired from the shoulder.” An overall length above 26 inches rules out AOW status, because at that length the weapon isn’t concealable.5Violence Policy Center. Mossberg Shockwave ATF Approval Letter What’s left is a plain GCA “firearm” with no NFA paperwork required.
Why 26 Inches Is the Number That Matters
The 26-inch overall length threshold is the single most important measurement for this classification. Below 26 inches, a weapon that doesn’t fit the rifle, shotgun, or handgun definitions is concealable enough to be an AOW. At 26 inches or above, it can sit in the non-NFA “other firearm” space.
Overall length is measured from the muzzle to the rearmost point of the weapon along a line parallel to the bore.6Bureau of Alcohol, Tobacco, Firearms and Explosives. National Firearms Act Handbook For firearms with folding or collapsible components, whether you measure extended or collapsed has been a moving target in ATF guidance. Anyone building or modifying a firearm near that line should verify current ATF measurement standards first. Dropping below 26 inches by accident can turn a legal firearm into an unregistered NFA weapon.
Why the Category Exists
The standard federal definitions leave a gap, and the “other firearm” label fills it. Under federal law:
- A rifle is designed to be fired from the shoulder and has a rifled bore.2Office of the Law Revision Counsel. 26 USC 5845 – Definitions
- A shotgun is designed to be fired from the shoulder and has a smooth bore.2Office of the Law Revision Counsel. 26 USC 5845 – Definitions
- A handgun has a short stock and is designed to be held and fired with one hand.7Office of the Law Revision Counsel. 18 USC 921 – Definitions
A weapon that was never manufactured with a shoulder stock was never “designed to be fired from the shoulder,” so it can’t be a rifle or shotgun no matter how long the barrel. A weapon that requires two hands to operate isn’t a handgun. Something like the Shockwave fails all three tests and lands in the residual category.
The Age Rule That Catches Buyers Off Guard
Federal law bars licensed dealers from selling any firearm “other than a shotgun or rifle” to anyone under 21.8Office of the Law Revision Counsel. 18 USC 922 – Unlawful Acts Because a non-NFA “other firearm” is legally neither a rifle nor a shotgun, the 21-year minimum applies at the dealer counter, the same age floor as a handgun. The Form 4473 instructions state this plainly: any firearm classified as “Other” cannot be transferred by a dealer to anyone under 21.1Bureau of Alcohol, Tobacco, Firearms and Explosives. Firearms Transaction Record – ATF Form 4473
An 18-year-old who walks into a gun store expecting to buy a Shockwave the way they’d buy a shotgun will be turned away. Frames and receivers also get checked as “Other” on the 4473, so the same restriction applies even when the frame will only ever be built into a long gun.
Registering an AOW
If a firearm is an AOW, you register it with the ATF using one of two forms:
- Form 1 if you are making the AOW yourself (including a modification like adding a vertical foregrip to a handgun that brings the overall length under 26 inches).
- Form 4 if you are transferring an existing AOW from another owner, including buying one from a dealer.
Both forms require fingerprint cards and a photograph for each responsible person on the application. If the AOW is held by a trust or other legal entity, every trustee or responsible person submits prints and photos. The applicant also sends a copy of the completed application to the chief law enforcement officer in their area as notification.9Bureau of Alcohol, Tobacco, Firearms and Explosives. ATF eForm 1 Application Requirements
The tax picture changed under a 2025 amendment. The making tax and transfer tax for AOWs are now both $0. Only machineguns and destructive devices still carry a $200 tax.10Office of the Law Revision Counsel. 26 USC 5811 – Transfer Tax The registration and application requirement still applies, though. Skipping the paperwork because no tax is due is exactly the mistake that turns into a felony charge.
Penalties for Getting It Wrong
Possessing an unregistered AOW, failing to pay any tax that applies, or making an NFA firearm without ATF approval are each federal offenses. A conviction carries up to 10 years in prison, a fine of up to $10,000, or both.11Office of the Law Revision Counsel. 26 USC 5871 – Penalties Someone who bolts a vertical foregrip onto a pistol without understanding the AOW rules has committed the same statutory offense as a trafficker of unregistered weapons.
Separately, certain people cannot lawfully possess any firearm at all, whether NFA-regulated or a plain GCA “other firearm.” Federal law bars possession by anyone convicted of a crime punishable by more than a year in prison, anyone subject to a qualifying domestic violence restraining order, anyone convicted of a misdemeanor crime of domestic violence, and other listed categories.12Bureau of Alcohol, Tobacco, Firearms and Explosives. Identify Prohibited Persons
State Law Doesn’t Follow Federal Classification
A firearm that’s legal at the federal level and in one state may be banned, reclassified, or subject to a permit in another. Some states define “shotgun” more broadly than federal law does, which can pull a Shockwave-type firearm into a restricted category. Others regulate by feature, banning pistol grips or particular barrel lengths regardless of what the ATF calls the weapon.13Bureau of Alcohol, Tobacco, Firearms and Explosives. State Laws and Published Ordinances – Firearms
Federal classification tells you whether the ATF wants paperwork. It tells you very little about whether you’ll have a problem at a state traffic stop. Check the laws of your state and any state you’ll travel through.
What About Pistol Braces
The ATF’s 2023 rule reclassifying many braced pistols as short-barreled rifles created confusion for the “other firearm” space, because braced firearms often occupied similar legal ground. Multiple federal courts struck down the rule, and the Department of Justice dropped its appeal in 2025. Braced pistols are not currently classified as short-barreled rifles, and the rule is unenforceable. Anyone who modified or surrendered a braced firearm in response to the rule should verify the current status of their specific configuration with a qualified attorney before making further changes.