How bump stocks change the way semi-automatic weapons are fired

(WASHINGTON) — Bump stocks, the firearm modification banned by the federal government until the U.S. Supreme Court struck down the regulation Friday, have generated controversy ever since the device was created over how they change the way semi-automatic weapons are fired.

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

Typically, semi-automatic weapons can discharge only one bullet per press of the trigger. However, with the bump stock attached, the user can use the motion and energy from the kickback as the weapon slides back and forth and rapidly fires more bullets.

A shooter can keep a finger still as the recoil and bump stock move the trigger back and forth, according to weapons experts.

A bump stock modified semi-automatic can fire 400 to 800 rounds per minute, experts said. By comparison, a fully automatic weapon, which is prohibited for civilian use, shoots 700-950 rounds per minute.

Gun control advocates have called for the device to be banned after it was used in several shootings, including the 2017 Las Vegas mass shooting where 58 people were killed and hundreds were wounded.

It is the deadliest mass shooting in American history.

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

More than 700,000 bump stocks were sold since 2009.

However, the policy change was challenged in court by Michael Cargill, a Texas gun owner and weapons store owner, who contended the federal government overstepped its authority and bump stocks did not render semi-automatics as automatics.

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

The Supreme Court’s conservatives agreed with Cargill Thursday and struck down the ban.

ABC News’ Devin Dwyer contributed to this report.