In a 6-3 vote, the Supreme Court ruled Friday that a federal ban on bump stocks, gun accessories that allow semiautomatic rifles to fire more quickly, is unlawful.

What is a bump stock?

A bump stock is a firearm accessory that allows a semi-automatic rifle to mimic the firing speed of a fully automatic weapon. It replaces the standard stock (the part of the gun that rests against the shooter’s shoulder) and enables the shooter to use the gun’s recoil to “bump” the trigger into the finger, which can significantly increase the rate of fire. This is achieved by allowing the entire weapon to slide back and forth rapidly within the stock, causing the trigger to reset and fire again with each bump.

Here are key points about bump stocks:

• Functionality: Bump stocks harness the recoil energy produced when the gun is fired, using it to move the firearm back and forth in the stock. This rapid movement allows for a higher rate of fire without the need for multiple trigger pulls.

• Legality: The legality of bump stocks has been a controversial topic. They were banned in the United States following a directive by the Trump administration after the 2017 Las Vegas shooting, which involved a shooter using rifles equipped with bump stocks to kill 58 people and injure hundreds more. The ban, enacted in December 2018, classified bump stocks as machine guns under federal law, thus prohibiting their possession, sale, or manufacture.

• Usage: While bump stocks do not convert a semi-automatic firearm into a fully automatic one, they allow for a similar rate of fire. Fully automatic weapons are tightly regulated and generally prohibited for civilian use in many places, making bump stocks a controversial workaround to these restrictions.

Ban challenged by Texas gun shop owner

Michael Cargill, a gun shop owner and gun rights advocate from Austin, Texas, sued the government over the ban. The 5th U.S. Circuit Court of Appeals sided with Cargill and the Biden administration appealed.

In its decision, the appeals court said that the government was seeking to ban bump stocks “by administrative fiat,” even though the ATF had previously permitted the devices.

The case was of high concern to gun rights advocates who pointed out that hundreds of thousands of Americans own bump stocks, relying on the government’s pre-2018 determinations that they are allowed.

Advocates also feared that if the ATF won the case, the agency would go after semi-automatic weapons.

Gun control groups call bump stocks a “unique danger to society.”

Likewise, the American Medical Association, whose members said they’ve seen first-hand the “enormous human carnage, destruction and chaos” caused by rapid-fire bullets, said firearms modified by bump stocks “have no place in a civilized society.”

Supreme Court justices disagree on decision

Bump stocks were a controversial discussion in the highest court. Justice Clarence Thomas, writing for the majority, stated that a firearm outfitted with the accessory does not qualify as a machine gun under federal law.

The ruling sparked a strong dissent from liberal Justice Sonia Sotomayor.

“When I see a bird that walks like a duck, swims like a duck, and quacks like a duck, I call that bird a duck,” she wrote in reference to bump stocks enabling semiautomatic rifles to operate like machine guns.

How many states have banned bump stocks?

As of now, there are 15 states that have banned bump stocks.

Those states are:

• Nevada

• California

• Washington

• Hawaii

• Minnesota

• New York

• New Jersey

• Vermont

• Rhode Island

• Massachusetts

• Connecticut

• Florida

• Delaware

• Maryland

• District of Columbia

— USA TODAY reporter Maureen Groppe contributed to this report.

This article originally appeared on Austin American-Statesman: What is a bump stock? Supreme Court strikes down ban on gun accessory