How Bump Stocks Change the Way Semi-Automatic Weapons Are Fired

Bump stocks, the modification of firearms banned by the federal government until the U.S. Supreme Court struck down the regulation Friday, have sparked controversy since the device's inception over how they change the way semi-automatic weapons are fired.

The modification replaces the rear of a semi-automatic rifle, which is typically held against the shoulder, and allows the weapon to slide back and forth.

Typically, semi-automatic weapons can only fire one bullet per trigger pull. However, with the stock attached, the user can use the motion and rebound energy as the weapon slides back and forth and quickly fires more bullets.

A shooter can hold a finger still while the recoil and stock move the trigger back and forth, according to gun experts.

A modified semi-automatic can fire 400 to 800 rounds per minute, experts say. For comparison, a fully automatic weapon, banned for civilian use, fires between 700 and 950 rounds per minute.

Gun control advocates called for a ban on the device after it was used in several shootings, including the 2017 Las Vegas mass shooting, where 58 people were killed and hundreds more others injured.

It was the deadliest mass shooting in American history.

A year later, President Donald Trump ordered the Justice Department to reclassify bump stocks to fall under the federal definition of “machine gun” and ban their sale, a dramatic change in their policy that allowed explicitly the sale and use of the modification.

More than 700,000 bump stocks have been sold since 2009.

However, the policy change was challenged in court by Texas gun owner and gun store owner Michael Cargill, who claimed the federal government had overstepped its authority and that stockpiles of replacement didn't make semi-automatics like automatics.

“An agency within the federal government can't go out and turn millions of people into criminals overnight or ban a product. We have to go to Congress to do it.” Cargill told ABC News in February.

Conservatives on the Supreme Court ruled in favor of Cargill on Thursday and overturned the ban.

ABC News' Devin Dwyer contributed to this report.