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I want to know about the legal definitions of automatic firearms in the US. I don't care about what people think an automatic firearm is, just about what legally qualifies one as such.

The only legal definition I was able to find online is this one:

The term “semiautomatic rifle” means any repeating rifle which utilizes a portion of the energy of a firing cartridge to extract the fired cartridge case and chamber the next round, and which requires a separate pull of the trigger to fire each cartridge.

(https://www.law.cornell.edu/uscode/text/18/921)

This, however, only defines semi-automatic rifles. It does not define, for example, semi-automatic handguns or fully automatic rifles.

  • There are multiple definitions of automatic weapons which are not identical under U.S. law. For example, there are definitions found in the criminal code, in international customs regulations and in the tax code (which governs excise taxes and licensing of certain firearms). I suspect those aren't the only ones. So, the purpose for which the definition is needed matters. – ohwilleke Nov 18 '17 at 3:20
  • Hmm, that's a weird thing. I would've assumed the same definitions to apply to taxation, ownership, customs, etc. I'm most interested in those regulating ownership. Are those federal definitions? – UTF-8 Nov 18 '17 at 12:21
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    You need to get over that assumption: in general, each statute will define its own terms, or rely on normal English usage (although the definition can refer to another statute). If a statute does not define a term, but an earlier one does, that might be persuasive as to what the legislators meant, but is not conclusive. – Martin Bonner supports Monica Nov 18 '17 at 14:06
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    Fully automatic weapons are called "machineguns" in US law. – cHao Nov 20 '17 at 17:52
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The legal definition you are probably looking for is machinegun rather than automatic weapon. It can be found in 26 U.S. Code § 5845, which in relevant part says:

The term “machinegun” means 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 term shall also include the frame or receiver of any such weapon, any part designed and intended solely and exclusively, or combination of parts designed and intended, for use in converting a weapon into a machinegun, and any combination of parts from which a machinegun can be assembled if such parts are in the possession or under the control of a person.

Historical notes; TLDR

The core of this definition dates back to the 1934 National Firearms Act (NFA) . The legislative history of that act includes testimony of Karl. T. Fredrick in front of the house ways and means committee.

Mr. Fredrick, then president of the National Rifle Association (NRA), had devoted years to the study of earlier firearms legislation. He criticized earlier legislative attempts to define machine guns, calling them wholly inadequate. The earlier legislative definition was:

"Machine gun", as used in this Act, means any firearm which shoots automatically more than twelve shots without reloading.

Mr. Fredrick expressed several concerns with this definition, particularly with regard to the phrase more than twelve shots without reloading. His propoed language was substantially broader:

A machine gun or submachine gun, as used in this act, means any firearm by whatever name known, loaded or unloaded, which shoots automatically more than one shot without manually reloading, by a single function of the trigger.

Note that neither of the later two quotes is current. Thus the core of the definition used today, was in fact provided by an officer of the National Rifle Association, but that definition has been changed in the process of clarifying and codifying the law.

  • So an automatic crossbow is a machine gun, legally speaking? – UTF-8 Oct 3 at 12:58
  • Interesting video, I found another one showing a Lego machinegun!. Out of curiosity, was that what motivated your original question? – Burt_Harris Oct 3 at 23:10
  • P.S. I'm not a lawyer: if interpreting how the law applies to a specific situation is important to you, you need a lawyer to opine. – Burt_Harris Oct 3 at 23:11
  • No, that video did not motivate me to ask this question. But that channel certainly played a role in my decision to ask this question because it often highlights (and profits from) the absurdities of German weapons law (German gun law is mingled with laws about other weapons). In particular, it profits off the fact that the German definition of a Schusswaffe (literally "shooting weapon"; not to be confused with a Feuerwaffe (firearm/"fire weapon") makes no sense whatsoever. I heard that American legal definitions make more sense in this regard, but then there was that whole bump stock debate. – UTF-8 Oct 4 at 11:52
  • I added some historical notes to the answer which you may find interesting. – Burt_Harris Oct 4 at 18:00
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Asking the difference between Semi-Automatic Rifles and Semi-Automatic Hand guns is like asking the difference between Cars with Automatic Transmissions and Trucks with Automatic Transmissions: There are going to be plenty of differences, even in the transmission mechanisms, but Automatic Transmission will provide the driver with the same features regardless of a vehicle (you do not need to change gears, for this analogy).

Automatic and Semi-Automatic only refer to the nature of how a gun fires and reloads. It has nothing to do with any other nature of the weapon or the amount of ammunition that be fed into the chamber before reload. The basic rule of thumb to go by is that:

1.) Automatic Guns are guns that, when the trigger is engaged, will fire until either the trigger is disengaged or all loaded ammo is spent, which ever happens first. These are illegal to own by civilians unless they were made prior to the date of the law going into effect (I believe 1987) and in those cases, they are extremely cost prohibited (limited availability and licensing both make a hefty price tag).

2.) Semi-automatic Guns are guns that, when the trigger is engaged, will fire exactly one bullet (if loaded) and will not fire until the trigger is disengaged and reengaged. However, no effort is required to load firing chamber with a new bullet if there are still available rounds in the clip/magazine whatever. This is the broadest term as it can refer to hand-guns (generally all handguns are semi-automatic) and rifles, some of which are civilian versions of military fully automatic weapons that are designed to be semi-automatic (again, cannot buy new Automatic weapons at all).

3.) Non-Automatic are guns that must be manually reloaded after firing all chambered rounds.

For classification purposes, there is no difference between semi-automatic hand guns and semi-automatic rifles from the point of view of what is semi-automatic. It's one trigger pull, one bullet fired, rinse and repeat. One problem with laws as written is that while it is illegal to modify a semi-automatic weapon to be full automatic, it is not illegal to add attachments that will effectively change the firing nature of the gun. The device known as a "bump stock" that was used to make the weapon used in the Vegas Strip shooting fully automatic was not (at time of writing) illegal because it is classified as an attachment, not a modification. This may change in the near future as it is a rare issue, though it is hitting some stone walls (from what I've gathered, it does allow people who suffer from nerve damage to fire a semi-automatic rifle.). This works by using the recoil of the gun to re-engage the trigger (disengaged by the recoil) without the shooter moving his or her finger, effectively simulating automatic fire (stressing once again, this is not changing the semi-automatic nature of the gun. It just lets physics of a fired weapon pull the trigger for you).

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    I know what fully automatic and semi-automatic firearms are. I know what bumpfire is. However, this question is about legal definitions (which can be different from the commonly used definitions). That's why I wrote the second sentence of the question. Besides: "generally all handguns are semi-automatic" is just wrong. But your definitions would definitely declare more firearms semi-automatic than usually are considered as such. For example, your definition would declare DA revolvers fully automatic and double-barrel shotguns semi-automatic. This is obviously incorrect. – UTF-8 Nov 20 '17 at 22:40
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    Like I said, I'm not a gun guy. Perhaps I wasn't clear but I did qualify that upon firing of a semi it reloads, which to my knowledge rules out double barrels because a shooter must manually reload them. Per legislation this is the difference between full and semi auto. And yes, I did some double takes at the list of guns that count as semi-auto. It's a lot more than non-gun folks expect. – hszmv Nov 21 '17 at 1:57

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