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Washington — The Supreme Court is convening Wednesday to consider a challenge to a Trump-era ban on bump stocks for firearms put in place in the wake of the 2017 mass shooting at a music festival in Las Vegas.

At issue in the case is whether a bump stock is a “machine gun” under federal law. The dispute is the second involving firearms that the Supreme Court will hear this term, but it does not involve the Second Amendment. Instead, the court fight raises a question of statutory interpretation and whether the Bureau of Alcohol, Tobacco, Firearms and Explosives had the authority to outlaw bump stocks.

Oral arguments in the case known as Garland v. Cargill will get underway at 10 a.m. ET. Audio of the arguments will stream live in the player above.

Machine guns have been largely prohibited since 1986, and are defined under federal law as “any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot, without manual reloading, by a single function of the trigger.” The definition also encompasses “any part designed and intended solely and exclusively, or combination of parts designed and intended, for use in converting a weapon into a machine gun.”

The legal fight over bump stocks

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.

Following the 2017 mass shooting at the Route 91 Harvest Music Festival, then-President Donald Trump directed the Justice Department to propose a new rule prohibiting all devices “that turn legal weapons into machine guns.”

ATF had determined on several occasions between 2008 and 2017 that bump stocks do not qualify as machine guns and are not regulated under federal law. But it reversed course following the massacre, during which the gunman used semi-automatic weapons outfitted with bump stocks. The devices allowed him to fire up to 1,000 rounds of ammunition in 11 minutes, according to the FBI, killing 58 people in the rampage — two others died later — and injuring roughly 500 more.

In its new rule, issued in December 2018, 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 took effect in March 2019, and those who already had bump stocks were required to destroy the devices or turn them into ATF. Violators who continued to possess bump stocks could face criminal penalties.

While the rulemaking process was underway, Michael Cargill, the man at the center of the case, bought two bump stocks in April 2018. He surrendered them to ATF after the ban was adopted, and then challenged the regulation in federal district court in Texas.

The district court sided with 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, determining 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. In a 13-3 decision in January 2023, the appeals court found that Congress must act to prohibit bump stocks. In a portion of the lead opinion written by Judge Jennifer Elrod, an eight-judge plurality said that bump stocks are clearly not covered by the statutory definition of machine gun. Twelve 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 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 the 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 argued 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.

Prelogar also urged the court to reject an interpretation of the law that would allow gun makers to circumvent the ban on machine guns.

“The contrary construction adopted by the Fifth Circuit would transform Congress’s longstanding restrictions on machineguns into a ‘Maginot Line, easily circumvented by the simplest maneuver,'” she wrote. “That implausible consequence further confirms what the natural reading of the statutory text instructs: A bump-stock device is a machinegun.”

But Cargill’s lawyers argued that bump stocks fall outside the definition of machine gun for two reasons: First, because the trigger of a semi-automatic rifle resets and must be reactivated by the shooter between every shot fired from a weapon equipped with a bump stock, the device doesn’t cause the gun to fire more than one shot “by a single function of the trigger;” and second, 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, they urged it to side with Cargill because of the rule of lenity. Cargill’s lawyers also noted that Congress could have acted itself to ban bump stocks, but didn’t.

“What Congress enacted was a statutory definition of ‘machinegun’ that turns on whether a weapon can fire more than one shot ‘automatically … by a single function of the trigger,” they said. “Whether that definition should be updated to encompass weapons that achieve the results of machineguns ‘through different technological means’ is a decision for Congress to make, not agencies or courts.”

A decision from the Supreme Court is expected by the end of June.