On March 26, 2019, every American who owned a bump stock, a rifle accessory that facilitates rapid firing, was suddenly guilty of a federal felony punishable by up to 10 years in prison. That did not happen because a new law took effect; it happened because federal regulators reinterpreted an existing law to mean something they had long said it did not mean.

On Wednesday, the U.S. Supreme Court considered the question of whether those bureaucrats had the authority to do that. The case, Garland v. Cargill, turns on whether bump stocks are prohibited under the "best reading" of the federal statute covering machine guns. While several justices were clearly inclined to take that view, several others had reservations.

The products targeted by the government are designed to assist bump firing, which involves pushing a rifle forward to activate the trigger by bumping it against a stationary finger, then allowing recoil energy to push the rifle backward, which resets the trigger. As long as the shooter maintains forward pressure and keeps his finger in place, the rifle will fire repeatedly. The "interpretive rule" at issue in this case, which was published in December 2018 and took effect three months later, bans stock replacements that facilitate this technique by allowing the rifle's receiver to slide back and forth.

Officially, the purpose of that rule was merely to "clarify" that bump stocks are illegal. According to the Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF), they always have been, although no one (including the ATF) realized that until 2018.

Federal law defines a machine gun as a weapon that "automatically" fires "more than one shot" by "a single function of the trigger." The definition also covers parts that are "designed and intended…for use in converting a weapon" into a machine gun.

During Wednesday's oral arguments, Principal Deputy Solicitor General Brian H. Fletcher maintained that a rifle equipped with a bump stock plainly meets the criteria for a machine gun. It "fires more than one shot by a single function of the trigger," he said, because "a function of the trigger happens when some act by the shooter, usually a pull, starts a firing sequence." An ordinary semi-automatic rifle, according to Fletcher, "fires one shot for each function of the trigger because the shooter has to manually pull and release the trigger for every shot." But "a bump stock eliminates those manual movements and allows the shooter to fire many shots with one act, a forward push."

Fletcher argued that a rifle with a bump stock also "fires more than one shot automatically, that is, through a self-regulating mechanism." After "the shooter presses forward to fire the first shot," he said, "the bump stock uses the gun's recoil energy to create a continuous back-and-forth cycle that fires hundreds of shots per minute."

Jonathan F. Mitchell, the attorney representing Michael Cargill, the Texas gun shop owner who challenged the bump stock ban, argued that Fletcher was misapplying both of those criteria. First, he said, a rifle equipped with a bump stock "can fire only one shot per function of the trigger because the trigger must reset after every shot and must function again before another shot can be fired." The trigger "is the device that initiates the firing of the weapon, and the function of the trigger is what that triggering device must do to cause the weapon to fire," he added. "The phrase 'function of the trigger' can refer only to the trigger's function. It has nothing to do with the shooter or what the shooter does to the trigger because the shooter does not have a function."

Second, Mitchell said, a rifle with a bump stock "does not and cannot fire more than one shot automatically by a single function of the trigger because the shooter, in addition to causing the trigger to function, must also undertake additional manual actions to ensure a successful round of bump firing." That process "depends entirely on human effort and exertion," he explained, because "the shooter must continually and repeatedly thrust the force stock of the rifle forward with his non-shooting hand while simultaneously maintaining backward pressure on the weapon with his shooting hand. None of these acts are automated."

Justices Elena Kagan and Ketanji Brown Jackson seemed eager to accept Fletcher's reading of the law, arguing that it is consistent with what Congress was trying to do when it approved the National Firearms Act of 1934, which imposed tax and registration requirements on machine guns. Although bump stocks did not exist at the time, they suggested, the law was meant to cover any firearm that approximated a machine gun's rate of fire.

According to Fletcher, "a traditional machine gun" can "shoot in the range of 700 to 950 bullets a minute," while a semi-automatic rifle with a bump stock can "shoot between 400 and 800 rounds a minute." As he conceded, however, the statute does not refer to rate of fire. "This is not a rate-of-fire statute," he said. "It's a function statute." To ban bump stocks, in other words, the ATF has to show that they satisfy the disputed criteria.

"It seems like, yes, that this is functioning like a machine gun would," Justice Amy Coney Barrett said. "But, you know, looking at that definition, I think the question is, 'Why didn't Congress pass…legislation to make this cover it more clearly?'"

Justice Neil Gorsuch made the same point. "I can certainly understand why these items should be made illegal," he said, "but we're dealing with a statute that was enacted in the 1930s, and through many administrations, the government took the position that these bump stocks are not machine guns." That changed after a gunman murdered 60 people at a Las Vegas country music festival in October 2017, and it turned out that some of his rifles were fitted with bump stocks.

The massacre inspired several bills aimed at banning bump stocks. Noting that "the ATF lacks authority under the law to ban bump-fire stocks," Sen. Dianne Feinstein (D–Calif.) said "legislation is the only answer." President Donald Trump, by contrast, maintained that new legislation was unnecessary. After he instructed the ATF to ban bump stocks by administrative fiat, the agency bent the law to his will. Noting that "the law has not changed," Feinstein warned that the ATF's "about face," which relied partly on "a dubious analysis claiming that bumping the trigger is not the same as pulling it," would invite legal challenges.

Feinstein was right about that, and one of those challenges resulted in the decision that the government is now asking the Supreme Court to overturn. In January 2023, the U.S. Court of Appeals for the 5th Circuit rejected the ATF's redefinition of machine guns.

"A plain reading of the statutory language, paired with close consideration of the mechanics of a semi-automatic firearm, reveals that a bump stock is excluded from the technical definition of 'machinegun' set forth in the Gun Control Act and National Firearms Act," 5th Circuit Judge Jennifer Walker Elrod wrote in the majority opinion. And even if that were not true, Elrod said, "the rule of lenity," which requires construing an ambiguous criminal statute in a defendant's favor, would preclude the government from punishing people for owning bump stocks.

Gorsuch alluded to Feinstein's prescient concerns about the ATF rule's legal vulnerability: "There are a number of members of Congress, including Senator Feinstein, who said that this administrative action forestalled legislation that would have dealt with this topic directly, rather than trying to use a nearly 100-year-old statute in a way that many administrations hadn't anticipated." The ATF's attempt to do that, he said, would "render between a quarter of a million and a half million people federal felons," even though they relied on guidance from "past administrations, Republican and Democrat," that said bump stocks were legal.

Justices Brett Kavanaugh and Samuel Alito also were troubled by that reversal's implications for people who already owned bump stocks. Fletcher tried to assuage those concerns.

"ATF made [it] very clear in enacting this rule that anyone who turned in their bump stock or destroyed it before March of [2019] would not face prosecution," Fletcher said. "As a practical matter," he added, "the statute of limitations for this offense is five years," meaning prosecutions of people who owned bump stocks before the rule took effect will no longer be possible a month from now. "We have not prosecuted those people," he said. "We won't do it. And if we try to do it, I think they would have a good defense based on entrapment by estoppel," which applies when someone follows official advice in trying to comply with the law.

"What is the situation of people who have possessed bump stocks between the time of the ATF's new rule and the present day or between the time of the new rule and the 5th Circuit decision?" Alito asked. "Can they be prosecuted?" Fletcher's answer: "probably yes." That prospect, Alito said, is "disturbing."

Kavanaugh wondered about gun owners who did not destroy or surrender their bump stocks because they did not know about the ATF's rule. "For prosecuting someone now," he asked, "what mens rea showing would the government have to make to convict someone?" Fletcher said the defendant would "have to be aware of the facts" that, according to the ATF's reinterpretation of the law, make bump stocks illegal. "So even if you are not aware of the legal prohibition, you can be convicted?" Kavanaugh asked. "That's right," Fletcher replied.

"That's going to ensnare a lot of people who are not aware of the legal prohibition," Kavanaugh said. "Why not require the government to also prove that the person knew that what they were doing…was illegal?"

Gorsuch mocked Fletcher's apparent assumption that gun owners can be expected to keep abreast of the ATF's edicts. "People will sit down and read the Federal Register?" he said to laughter. "That's what they do in their evening for fun. Gun owners across the country crack it open next to the fire and the dog."

Maybe not, Fletcher admitted, but the publicity surrounding the ban and the legal controversy it provoked probably brought the matter to many people's attention. "I agree not everyone is going to find out about those things," he said, "but we've done everything the government could possibly do to make people aware."

Beyond the unfairness to gun owners who bought products they quite reasonably thought were legal, the ATF's about-face lends credibility to the complaint that its current interpretation of the law is misguided. If the ATF was wrong before, how can we be confident that it is right now?

According to the agency's new understanding of the statute, Mitchell noted, "function of the trigger" hinges on what the shooter is doing. But "function is an intransitive verb," he said. "It can't take an object grammatically. It's impossible. The trigger has to be the subject of function. It can't be the object."

Gorsuch picked up on that point, noting that the government had likened "function of the trigger" to "a stroke of a key or a throw of the dice or a swing of the bat." But "those are all things that people do," he said. Since function is an intransitive verb, "people don't function things. They may pull things, they may throw things, but they don't function things."

Gorsuch noted that the ATF is relying on "a very old statute" designed for "an obvious problem" posed by gangsters like Al Capone armed with machine guns that fired repeatedly "with a single function of the trigger—that is, the thing itself was moved once." Maybe legislators "should have written something better," he said. "One might hope they might write something better in the future. But that's the language we're stuck with."

What about the ATF's claim that a rifle equipped with a bump stock shoots "automatically"? Fletcher conceded that "an expert" can bump-fire a rifle "without any assistive device at all" and that "you can also do it if you have a lot of expertise by hooking your finger into a belt loop or using a rubber band or something else like that to hold your finger in place." But he added that "we don't think those things function automatically because the definition of 'automatically'" entails "a self-regulating mechanism."

As the government sees it, a shooter creates such a mechanism by using a bump stock, notwithstanding the "manual actions" that Mitchell highlighted. "There's nothing automatic about that," Mitchell argued. "The shooter is the one who is pushing. It's human effort, human exertion. Nothing automatic at all about this process."

Barrett asked Fletcher how the ATF would treat an elastic "bump band" marketed as an accessory to facilitate rapid firing. "Why wouldn't that then be a machine gun under the statute?" she wondered. "We think that's still not functioning automatically because that's not a self-regulating mechanism," Fletcher replied.

Mitchell, by contrast, argued that Barrett's hypothetical product and a bump stock are "indistinguishable when it comes to 'automatically.'" Bump firing with either involves "a manual action undertaken entirely by the shooter," he said. "There is no automating device….It is all being done by the shooter."

Justice Sonia Sotomayor, who was sympathetic to Fletcher's argument, nevertheless implied that the legal status of bump stocks might not be as clear as the government suggests. "The back-and-forth here leads me to believe that at best there might be some ambiguity," she said. But if the statute is in fact unclear, the 5th Circuit said, the ambiguity should be resolved in a way that protects gun owners from prosecution for a crime invented by bureaucrats.

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