Congress Should Regulate Tools That Bypass Federal Machine Gun Law

October 10, 2017

Bottom Line: Congress should act to close gaps that allow widely available firearm accessories to effectively convert semiautomatic firearms into fully automatic weapons. While machine guns have been tightly regulated since the 1930s, gun manufacturers have exploited gaps in the law to sell bump stocks and other devices that achieve nearly fully automatic rates of fire without technically converting a firearm into a machine gun.

With the gun lobby on the march in Washington, Congress must focus on rejecting dangerous legislation to gut state concealed carry laws (so-called “concealed carry reciprocity”) and to remove protections that keep gun silencers from falling into the wrong hands. But to stem the gun violence epidemic, Congress must also be proactive—regulating these dangerous devices, and taking the more critical steps to require background checks on all firearm sales.

Fully automatic weapons, more commonly known as machine guns, are highly regulated under federal law.

  • A fully automatic weapon, or machine gun, fires multiple shots with a single compression of the trigger; whereas semiautomatic firearms fire one shot per trigger compression.
  • Federal law tightly regulates machine guns and their parts and prohibits the manufacture of new machine guns for civilian use.26 U.S.C. § 5845; 18 U.S.C. § 922(o).
  • Machine guns that were in circulation as of May 1986 can be legally possessed, but they must be registered with ATF and transferred in accordance with federal law.18 U.S.C. § 922(o).
  • To receive a machine gun or a machine gun part, a person must generally submit to an extensive background investigation process that includes a background check and notification to law enforcement.See generally, Bureau of Alcohol, Tobacco, Firearms and Explosives, “National Firearms Act Handbook,” available at:
  • Given that there are relatively few machine guns in circulation, federally compliant machine guns can cost more than $10-20,000.The Trace, “AR-15 Lovers Are Getting Fully Automatic Thrills With Barely Legal Gadgets,” Nov. 16, 2015, available at:

But widely available, legal and inexpensive accessories allow for semiautomatic firearms to be effectively converted into machine guns.

  • The relative unavailability of machine guns has led to a growing market for firearm accessories that can simulate the rate of fire of fully automatic firearms.
  • These devices are legal because they exploit a gap in federal law.
    • The definition of machine gun under federal law includes devices that automatically fire more than one shot with a “single function” of the trigger.26 U.S.C. § 5845; 18 U.S.C. § 921(a)(23).
    • Because these devices speed up the rate at which a trigger is pulled, rather than allowing multiple shots with a single pull, they are not considered machine guns under the law.
  • “Bump Stocks.” A “bump stock” is a device that harnesses the recoil of a semiautomatic
    firearm to fire several shots in succession, mimicking automatic fire.
    • Bump stocks enable a shooter to discharge 400-800 rounds per minute. One distributor, Slide Fire Solutions, advertised that their device enabled a shooter to unleash 100 rounds in 7 seconds.6 Slate, “This Single Legal Add-On Lets an AR-15 Fire 900 Rounds Per Minute,” Jan 7, 2013, available at: e.html.
    • By contrast, the rate of fire for traditional semiautomatic weapons is primarily governed by the ability of the shooter to pull the trigger. Average shooters can usually fire 45-60 rounds per minute while maintaining some degree of accuracy.
    • These stocks are relatively inexpensive, costing anywhere from about $100-400.
  • “Positive Reset Triggers.” A positive reset trigger achieves high rates of fire by using the force from the rifle to quickly reset the trigger, allowing for multiple shots in rapid succession.See generally, Tac-Con Fire Control, Inc., ATF Compliance Letter, available at:
  • Other devices. There are a range of other devices—including cranks and triggers—that can vastly increase a semiautomatic firearm’s rate of fire and are widely available for a cost of $50 to a few hundred dollars.See e.g., Two Z Precision, “Gat Crank”, available at:

These devices are designed specifically to circumvent the strict controls placed on fully automatic firearms and they have no sporting purpose.

  • ATF generally reviews new products to determine if they might run afoul of federal law—but this has triggered a “cat and mouse” game where manufacturers try to exploit technical configurations to get around federal law.
  • In 2006, ATF disapproved one such device—capable of firing 650 rounds per minute—determining it was appropriately classified as a machine gun under federal law.ATF Ruling 2006-2, Dec. 13, 2006, available at: But that decision simply inspired the creation of a new round of devices, many of which have been classified by ATF as outside of the machine gun law.
  • The companies that manufacturer these devices proudly advertise their products as ATF-approved
    and note that they are not subject to federal law.

    • One “bump stock” manufacturer boasted about their device, “Did you know that you can do simulated full-auto firing and it is absolutely legal?”Bump Fire Systems, available at:, accessed Oct. 3, 2017.
    • One positive reset trigger manufacturer advertises its trigger as being compliant with federal law. Marketing and press materials describe at length how the trigger achieves high rates of fire—describing it as nearly fully automatic—while stating that it is legal under federal law.See, Tac-Con Fire Control, Inc.,; See also,
  • These devices have no sporting purpose. They are specifically designed to mimic fully automatic weapons and discharge as many shots per second as possible.

Congress should act swiftly to regulate or prohibit these items—and should resist new attempts to build loopholes into the law.

  • Federal law has regulated machine guns for over 80 years. Congress should act swiftly to prevent manufacturers from subverting federal law by creating devices that are designed essentially to convert legal firearms into machine guns.National Firearms Act of 1932, Public Law 73 P.L. 474.
  • Congress should also resist efforts by the NRA to loosen restrictions on other weapons regulated under federal law, like gun silencers. And our legislators should reject the gun lobby’s top priority, “concealed carry reciprocity,” which would override state gun laws on who can carry concealed firearms in public and allow people who have never obtained a permit or firearm safety training to carry concealed guns in every state in the country.