Bump stocks and other similar devices are marketed to shooters seeking to convert their weapon to simulate the rapid, continuous fire of an automatic firearm while using a semi-automatic gun. In October 2017, a gunman in Las Vegas used multiple bump stock devices to convert semi-automatic rifles into weapons that fired 9 shots per second. He used those weapons to carry out the deadliest mass shooting attack in modern history. These devices are currently legal in most states and are not regulated by the federal government.

Download State Legislative Toolkit: Addressing Bump Stocks as a PDF. 

Why Are Bump Stock Devices Legal?

For over eight decades, federal law has strictly regulated the sale, manufacture, and possession of machine guns, which are fully automatic firearms that continue to fire bullets as long as the trigger is pressed and ammunition is available. This continuous fire feature makes machine guns particularly lethal if they are in the wrong hands.

Under existing federal law, a person may only transfer, acquire, or own a machine gun that was lawfully manufactured and owned prior to May 19, 1986. The transfer must also be approved by the Bureau of Alcohol, Tobacco, Firearms & Explosives (ATF) subject to a thorough background check, registration, and other requirements as outlined under the National Firearms Act. These restrictions have been highly effective and help explain why fully automatic machine guns are very rarely used in crimes or mass shooting attacks.

In recent years, however, the gun industry has marketed bump-stocks and similar devices, such as trigger cranks, that attach to a semi-automatic firearm and are designed to accelerate the weapon’s rate of fire to rates approaching automatic machine gun fire. These devices skirt the limits of federal law because federal restrictions on fully automatic “machine guns” only apply to weapons that automatically fire multiple rounds with “a single function of the trigger.” This determination is based on how the gun is fired, as opposed to the rate at which a gun is fired. Bump stocks and similar devices enable a shooter to repeatedly engage the firearm’s trigger with extreme speed, instead of firing multiple rounds with a single pull. As a result, they are legal and entirely unregulated at the federal level.

Bump-fire Devices

Bump-fire devices replace a rifle’s standard shoulder stock (the part of the weapon that is held against the shooter’s shoulder) with a stock that allows the weapon to slide (or “bump”) back and forth very rapidly between the shooter’s shoulder and trigger finger. By harnessing the weapon’s recoil, the bump stock causes the weapon’s trigger to be engaged many times faster than a human could otherwise fire.

Trigger Cranks

Trigger cranks attach to a firearm’s trigger guard and similarly enable a shooter to fire a semi-automatic weapon very rapidly by simply rotating a crank that repeatedly engages the trigger much faster than a human could otherwise fire.

Most states restrict machine guns using the same statutory definitions as federal law. As a result, bump stocks and similar devices are legal and available for sale without any background check or sale record in nearly every state.

Firearms that fire ammunition more rapidly can be particularly lethal: in 1993, a shooter killed eight people and wounded six others at a law firm at 101 California Street in San Francisco by using a TEC DC-9 assault pistol equipped with a Hellfire trigger activator. This led to the establishment of the Law Center to Prevent Gun Violence, now Giffords Law Center.

Legislation to Protect Americans from Bump Stocks & Similar Devices

Lawmakers interested in acting to protect their communities from bump stock devices can look to other legislation that has been enacted or introduced at the state and federal level. For example:


California is one of the few states with a law against the sale, transfer, manufacture, and possession of bump stocks, trigger cranks, and similar devices. This 25-year old state law applies to “multiburst trigger activators,” which are defined to include either of the following:

  • A device designed or redesigned to be attached to a semiautomatic firearm, which allows the firearm to discharge two or more shots in a burst by activating the device; [or]
  • A manual or power-driven trigger activating device constructed and designed so that when attached to a semiautomatic firearm it increases the rate of fire of that firearm.


Massachusetts legislators acted in the wake of the Las Vegas massacre to enact legislation that specifically prohibits the sale, possession, and manufacture of bump stocks and trigger crank devices, which are defined as:

  • Bump stock—Any device for a weapon that increases the rate of fire achievable with such weapon by using energy from the recoil of the weapon to generate a reciprocating action that facilitates repeated activation of the trigger.
  • Trigger crank—Any device to be attached to a weapon that repeatedly activates the trigger of the weapon through the use of a lever or other part that is turned in a circular motion; provided, however, that “trigger crank” shall not include any weapon initially designed and manufactured to fire through the use of a crank or lever.

In 2013, US Senator Dianne Feinstein introduced federal legislation, which would have generally restricted “any part, combination of parts, component, device, attachment, or accessory that is designed or functions to accelerate the rate of fire of a semiautomatic rifle but not convert the semiautomatic rifle into a machine gun.”

The most effective legislation to address bump stocks and similar devices should apply to both firearm accessories and parts that significantly increase a semi-automatic firearm’s rate of fire beyond the standard rate of fire for a semi-automatic firearm.

About Giffords

Giffords is a nonprofit organization dedicated to saving lives from gun violence. Led by former Congresswoman Gabrielle Giffords and her husband, Navy combat veteran and retired NASA astronaut Captain Mark Kelly, Giffords inspires the courage of people from all walks of life to make America safer.

About SiX Action

SiX Action is an independent, nonpartisan 501(c)(4) strategy and advocacy organization that is committed to achieving progressive change at the state level, while defending against efforts to move our country backward. SiX Action works with our sister 501(c)(3) organization, the State Innovation Exchange (SiX), a national resource and strategy center. For more information, visit sixaction.org and stateinnovation.org.