Definition of automatic firearms in the US
<p>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.</p>

<p>The only legal definition I was able to find online is this one:</p>

<blockquote>
  <p>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.</p>
</blockquote>

<p>(<a href="https://www.law.cornell.edu/uscode/text/18/921" rel="nofollow noreferrer">https://www.law.cornell.edu/uscode/text/18/921</a>)</p>

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

<p>The legal definition you are probably looking for is <strong>machinegun</strong> rather than <em>automatic weapon</em>.   It can be found in <a href="https://www.law.cornell.edu/uscode/text/26/5845" rel="nofollow noreferrer">26 U.S. Code § 5845</a>, which in relevant part says:</p>

<blockquote>
  <p>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.</p>
</blockquote>

<p><strong>Historical notes; TLDR</strong>  </p>

<p>The core of this definition dates back to the <strong>1934 National Firearms Act (NFA)</strong> .   The legislative history of that act includes <a href="http://www.alternatewars.com/Politics/Firearms/NFA_Hearings_HR-9066.pdf" rel="nofollow noreferrer">testimony of Karl. T. Fredrick in front of the house ways and means committee.</a>   </p>

<p>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 <em>wholly inadequate</em>. 
  The earlier legislative definition was:</p>

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

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

<blockquote>
  <p>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.</p>
</blockquote>

<p>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.</p>

<p>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).</p>

<p>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:</p>

<p>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).</p>

<p>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).</p>

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

<p>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).</p>

united-states definition firearms