Following the October 1, 2017, Las Vegas, NV, attack, there has been significant congressional interest in "bump-fire" stocks that can assist a person in firing a semiautomatic rifle repeatedly, sometimes at rates usually associated with fully automatic machineguns. It has been reported that the assailant in this attack had 12 semiautomatic rifles outfitted with "bump-fire" stocks. The terms "bump-fire" and "slide-fire" are often used interchangeably.
Under the Gun Control Act of 1968 (GCA), a semiautomatic rifle is defined as:
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 (18 U.S.C. § 921(a)(28)).
Semiautomatic hunting rifles have been available to civilians in the United States since the 1890s.
Under the 1934 National Firearms Act (NFA), a machinegun is defined as, among other things,
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 (26 U.S.C. § 5845(b)).
In 1986, Congress prohibited the possession or transfer of machineguns that were not lawfully possessed before the President signed the bill on May 19, 1986 (18 U.S.C. § 922(o)).
The Department of Justice's Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) has evaluated and classified an unknown number of "bump-fire" or "slide-fire" stocks through "private letter rulings" and at least one published ATF ruling.
According to ATF, private letter rulings are a means to make specific classification decisions about firearms and ammunition. The firearms industry and individuals may submit items to ATF to make determinations for classification (e.g., a "part," a "firearm," a "machinegun," a "silencer," etc.). These submissions are not required, but are encouraged by ATF to ensure compliance with federal law. ATF makes a classification determination based on the most current laws and regulations at the time of submission and on the results of a physical examination and testing of that specific item. ATF maintains that private letter rulings are proprietary and does not release these letters without written approval from the recipient.
As opposed to private letter rulings, ATF also publishes letter rulings signed by the ATF Director. ATF rulings are intended to promote uniform application of the laws and regulations it administers, and these rulings apply retroactively unless otherwise indicated and may be used as precedent.
In November 2003 and January 2004, ATF initially determined, through private letter rulings, that a "bump-fire" system, known as the Akins Accelerator, was not regulated as a firearm under either the NFA or GCA in private letter rulings according to an appellant brief submitted in federal court. On December 13, 2006, however, ATF reversed its determinations by publishing a rule, ATF Rul. 2006-2, which reclassified bump-fire systems like the Akins Accelerator as a machinegun, because it was equipped with a "coiled spring" and initiated automatic fire with a single trigger pull. William Akins, the inventor of the Akins Accelerator, appealed the ATF ruling in federal court. In an unpublished decision, the U.S. Court of Appeals for the Eleventh Circuit upheld ATF's decision (see Akins v. United States, 312 F. App'x 197 (11th Cir. 2009)).
Other manufacturers submitted modified "bump-fire" or "slide-fire" stocks that did not include a "coiled spring" or similar mechanisms to the ATF for classification. One manufacturer made a private letter ruling dated June 7, 2010, available on the Internet. In this letter, ATF concluded the following:
The stock has no automatically functioning mechanical parts or springs and performs no automatic mechanical function when installed. In order to use the installed device, the shooter must apply constant forward pressure with the non-shooting hand and constant rearward pressure with the shooting hand. Accordingly, we find that the "bump-stock" is a firearm part and is not regulated as a firearm under [the] Gun Control Act or the National Firearms Act.
In an October 1, 2012, edition of Shotgun News, one author described the technique of using a bump-fire stock:
When you wish to bump fire, you shoulder the rifle and it slides to the rear in the stock. Next, when you are aiming at a safe target and are ready to fire, you place your finger inside the trigger guard and rest it against the extension on the left side. Holding your trigger finger in place, you then merely push slightly forward with your left hand, maintaining forward pressure.
As you push forward, the rifle slides forward slightly in the stock, the trigger comes in contact with your finger and is depressed firing the rifle. The rifle's recoil then moves it slightly to the rear in the stock, allowing the trigger to reset before the forward pressure from your left hand slides it back forward, and the cycle repeats. ("SlideFire Stock: The Easy Way to Become a Bump-Fire Bandit," by David M. Fortier, pp. 26-31.)
It is noteworthy that some individuals can achieve bump-firing without a customized stock through trial and error. Nevertheless, these customized stocks arguably assist individuals with bump-firing, allowing them to sustain continuous semiautomatic rifle fire at rates that could not normally be sustained without a greater amount of practice.
From the ATF rulings discussed above, it appears that bump-fire stocks with "coiled springs" and that initiate automatic fire with a single trigger pull are regulated as firearms under the NFA; however, modified stocks without any "automatically functioning mechanical parts or springs" are not similarly regulated. Some Members of Congress have called on ATF to reevaluate its bump-fire stock determinations, while other Members have introduced legislation to prohibit such devices (S. 1916, H.R. 3947, and H.R. 3999).