The Supreme Court is weighing a Trump-era ban on bump stocks for guns. Here's what to know.

Washington — The Route 91 Harvest Music Festival in Las Vegas was becoming an annual tradition for Geena Marano Springmann. She had attended the three-day country music event with her best friend in 2016, and they returned the next year with Springmann's oldest sister.

But that year, as singer Jason Aldean was performing around 10 p.m., Springmann recalled hearing what first sounded like fireworks, but soon realized were gunshots, raining down from a window on the 32nd floor of a hotel and casino across the street. After being shielded by her sister, Marisa Marano, for several minutes, the women ran to a casino on the Las Vegas Strip. Springmann texted her mother to tell her that there was a shooting at the concert, they were running and she loved her.

Fifty-eight people were killed in the rampage — two others died later — and roughly 500 were wounded. The October 2017 massacre is the deadliest mass shooting in modern U.S. history, and it served as the catalyst for federal action that is now under review by the Supreme Court. 

The gunman had used semi-automatic weapons outfitted with so-called bump stocks, devices that allowed him to fire up to 1,000 rounds of ammunition in 11 minutes, according to the FBI. President Donald Trump's administration took steps swiftly following the shooting to ban the devices, issuing a rule that clarified that bump stocks are "machine guns" as defined by federal law. 

The high court heard arguments on Wednesday in a challenge brought in Texas in 2019 over the ban.

"I know the personal impact of the device, knowing that it could fire 800 bullets per minute, turning an assault weapon into a machine gun, essentially," Springmann, 29, told CBS News. "I know the devices are on the market, I know assault weapons exist, and I have seen firsthand the damage they can do. That night, what we went through was so horrific, I don't want anyone to experience that."

What is a bump stock?

A bump stock is an attachment that replaces a semi-automatic rifle's standard stock, the rear portion of the gun that rests on the shooter's shoulder. It allows the rest of the gun to move back and forth while the stock stays in place, and includes a finger rest that keeps the trigger finger still. When the gun is fired and the shooter applies forward pressure on the barrel, the rifle recoils back into the stock and bounces forward again, "bumping" the trigger into the finger and firing another round. The device allows the shooter to fire far more rapidly than is possible with a standard stock.

Though it has attracted input from a slew of organizations on both sides of the gun rights debate, the case before the Supreme Court, known as Garland v. Cargill, is not about the Second Amendment. Rather, at its heart lies a question of statutory interpretation, namely whether the Bureau of Alcohol, Tobacco, Firearms and Explosives had the authority to outlaw bump stocks.

"It's really just about who decides, and it really needs to be Congress that decides, not the ATF that decides," said Mark Chenoweth, president and chief legal officer of the New Civil Liberties Alliance, which is behind the challenge to the Trump-era ban. The power to write criminal laws is "Congress' alone," Chenoweth said.

The justices will consider whether a bump stock is a "machine gun" as defined by the National Firearms Act. Under the law, a machine gun is "any weapon which shoots ... automatically more than one shot, without manual reloading, by a single function of the trigger." It also includes parts used for "converting a weapon into a machine gun."

Federal appeals courts have reached different conclusions about whether the Trump-era ban on bump stocks is lawful. ATF itself had determined on numerous occasions between 2008 and 2017 that bump stocks do not qualify as machine guns and are not regulated under federal law.

But the agency reversed its position after the Las Vegas shooting. Trump directed the Justice Department to propose a new rule prohibiting all devices "that turn legal weapons into machine guns."

ATF determined that a rifle equipped with a bump stock qualifies as a machine gun, in part because when a shooter pulls the trigger, it initiates a firing sequence that produces more than one shot. The sequence is "automatic," the agency found, "because the device harnesses the firearm's recoil energy as part of a continuous back-and-forth cycle that allows the shooter to attain continuous firing after a single pull of the trigger."

The final rule from the Trump administration was issued in December 2018. Those who already had bump stocks were required to destroy the devices or turn them into ATF before the ban took effect in March 2019. Violators who continued to possess bump stocks could face criminal penalties.

Challenging the bump stock ban

While the rulemaking was underway, Michael Cargill, the man at the center of the case, purchased two bump stocks in April 2018. He turned them into ATF after the final rule was adopted, and one day later, challenged the ban in federal district court in Texas.

The district court ruled in favor of ATF, finding that the bureau properly classified rifles outfitted with bump stocks as machine guns. A three-judge panel of the U.S. Court of Appeals for the 5th Circuit upheld the lower court's decision, concluding that "bump stocks qualify as machine guns under the best interpretation of the statute."

But the full slate of judges on the 5th Circuit struck down the regulation, ruling 13-3 in January 2023 that Congress must act to prohibit bump stocks. In a portion of the lead opinion written by Judge Jennifer Elrod, an eight-judge plurality found that bump stocks are clearly not covered by the statutory definition of machine gun. But 12 judges sided with Cargill and concluded that the definition of machine gun was ambiguous enough to invoke a legal principle known as the rule of lenity, which requires courts to interpret unclear criminal laws in a way that's most favorable to the defendant.

Other federal appeals courts have also considered the legality of the bump stock regulation. The U.S. Court of Appeals for the 6th Circuit also applied the rule of lenity and sided with bump stock owner Scott Hardin, writing that "because the relevant statutory scheme does not clearly and unambiguously prohibit bump stocks, we are bound to construe the statute in Hardin's favor."

But a unanimous three-judge panel on the U.S. Court of Appeals for the District of Columbia Circuit let the rule stand, finding that a bump stock is a machine gun under federal firearms law because "under the best interpretation of the statute," the device is "a self-regulating mechanism that allows a shooter to shoot more than one shot through a single pull of the trigger."

In April 2023, the Biden administration appealed the 5th Circuit's ruling to Supreme Court, arguing that a semi-automatic rifle equipped with a bump stock satisfies the definition of a machine gun under federal law. The Justice Department also asked the Supreme Court to review the 6th Circuit's decision, while the gun owners who brought the D.C. case appealed their adverse ruling to the high court last summer.

The Supreme Court case

Lawyers for Cargill have argued that bump stocks fall outside the definition of machine gun for two reasons. First, since the trigger resets and must be reactivated between every shot from a weapon with a bump stock, the device doesn't cause the gun to fire more than one shot "by a single function of the trigger." Secondly, bump stocks do not enable a rifle to fire automatically, because the shooter has to engage in ongoing manual actions after activating the trigger.

If the Supreme Court finds that the language of the statute is ambiguous, Cargill's attorneys urged the justices to side with him because of the rule of lenity.

"The point of the rule of law is that you're supposed to be able to define the law in such a way that people can avoid the cut of the law ahead of time, particularly criminal law," Chenoweth said. "You want the law to be defined so that people can avoid violating it, and if you allow agencies to change the meaning of the law after people have already acted, then they don't have an opportunity to avoid the cut of the law."

But gun violence prevention advocacy groups are urging the justices to allow the bump stock ban to stand, arguing in a friend-of-the-court brief that the devices aim to turn semi-automatic rifles into machine guns. When a rifle is equipped with the device, a coalition of gun violence prevention groups said, the shooter only needs to pull the trigger once and the gun will fire continuously, as long as the shooter keeps his trigger finger stationary and maintains forward pressure on the barrel. 

They also told the court that over the last 100 years, Congress has repeatedly expanded the definition of "machine gun" to thwart manufacturers' efforts to get around the federal ban. The ATF rule, the groups wrote, is consistent with the plain-text reading of the law and furthers Congress' decision to bar devices that turn semi-automatic firearms into machine guns.

"The bump stock rule is simply common sense," Billy Clark, senior litigation attorney at the Giffords Law Center to Prevent Gun Violence, said during a call with reporters about the case. "Bump stocks turn semi-automatic rifles into fully automatic machine guns. That is their sole purpose. Congress has entrusted ATF with the ability to protect the public from technological advancements like bump stocks, and ATF acted squarely within its authority when it issued the bump stock rule."

The Biden administration is urging the Supreme Court to reverse the 5th Circuit's decision and allow the bump stock restriction to stand, arguing that ATF's interpretation of the definition of machine gun is the correct and best one.

"Rifles equipped with bump stocks, like conventional machine guns, are dangerous and unusual weapons," Solicitor General Elizabeth Prelogar, who represents the government before the high court, wrote in a filing.

Springmann said she can still remember vividly the night that was punctuated by the sound of gunfire more than six years ago and is living with the "invisible scars" of the shooting. She recalled crouching into the fetal position at a football game after a cannon was fired, and reminds herself that fireworks are just fireworks.

"It's terrifying to imagine a device can do that much damage and continue to be accessible to people if it gets into the wrong hands," Springmann said.

A decision from the court is expected by the end of June.