As of midnight on Tuesday, owners of "bump-stock-type devices" became felons, subject to a maximum penalty of 10 years in federal prison and a $250,000 fine. That's a pretty nasty surprise for anyone who bought these products, which were repeatedly declared legal by the Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF) during the Obama administration. It is especially unsettling because the law has not changed since the agency made that determination.

What happened instead is that the ATF, under orders from Donald Trump, reinterpreted the law to reach a diametrically opposed conclusion that is contrary to the plain meaning of the relevant statutes. Critics of presidential power grabs, regardless of how they feel about bump stocks or gun control more generally, should be troubled by this blatant usurpation of congressional authority.

Bump stocks are accessories that facilitate a rifle shooting technique in which recoil energy causes the gun to slide backward after a round is fired, resetting the trigger. By maintaining forward pressure on the rifle, the shooter bumps the trigger against his stationary finger, which fires another round, causing the rifle to slide backward again, and so on. Until October 1, 2017, the technique, which increases the rate of fire but reduces accuracy, was of interest mainly to gun enthusiasts, the companies that sold bump stocks, and the ATF officials who approved their sale. But after a gunman equipped with multiple rifles and bump stocks murdered 58 people in Las Vegas, Trump promised to ban the devices by administrative fiat.

The ATF rule that took effect this week accomplishes that trick by reclassifying bump stocks as illegal machine guns. The problem is that federal law defines a machine gun 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." A rifle with a bump stock does not fire automatically, since the shooter has to push the rifle forward while keeping his finger in position, and it does not fire more than one round by a single function of the trigger, since the trigger has to be reset before another round can be fired. That is why the ATF concluded, over and over again, that bump stocks are not machine guns. That is why, even after the Las Vegas massacre, both supporters and opponents of a bump stock ban agreed that it could be accomplished only by a new act of Congress.

Trump, who has a habit of ignoring inconvenient laws, was undeterred. He instructed the Justice Department, which includes the ATF, to come up with a legal rationale for the ban he had already decided to impose. The DOJ complied by defining "a single function of the trigger" as "a single pull of the trigger," defining pull to exclude what happens during bump fire, and treating the shooter as part of the rifle mechanism, ignoring his active participation in the process so that the gun could be said to fire "automatically."

Last month, responding to lawsuits by bump stock owners and gun rights groups, U.S. District Judge Dabney Friedrich concluded that the ATF had acted within its authority by reinterpreting "single function of the trigger" and "automatically," terms that he deemed "ambiguous." The plaintiffs, urging the U.S. Court of Appeals for the D.C. Circuit to overturn that ruling, vigorously and persuasively contest Friedrich's characterization of those terms.

"A 'single function of the trigger' involves the mechanical movement of the mechanism that constitutes the trigger," they note. "It is complete when the trigger traverses its range of motion and initiates the firing sequence. A separate function occurs when the trigger is released and returns to its starting point to reset….Regardless whether the shooter 'pulls' their finger against the trigger or pushes the firearm and trigger forward against a stationary finger, neither the operation of the trigger's component parts nor the operation of the firearm vary. Each round discharged is the result of a single function of the trigger initiated by the manual act of putting pressure on the reset trigger."

The plaintiffs also point out that the word automatically "means by mechanical process without further human intervention," so shooting automatically "means to continue to fire without further human action beyond a single function of the trigger." Bump fire, by contrast, is "mediated by the human intervention of repeatedly forcing the gun body and trigger forward to reengage the trigger after it has been returned to its starting position and has reset."

Accepting the ATF's counterintuitive definition of these terms "yields absurd results," the appeal brief notes, since the statutory definition of machine gun also includes "parts designed and intended" to convert a weapon into a machine gun and "any combination of parts from which a machinegun can be assembled if such parts are in the possession or under the control of a person." The ATF concedes that "individuals wishing to replicate the effects of bump-stock-type devices could also use rubber bands, belt loops, or otherwise train their trigger finger to fire more rapidly." The implication is that any semi-automatic rifle, if owned by someone who also has rubber bands, belt loops, or fingers, could be considered a machine gun. Although the ATF has said it does not intend to follow through on that logic, such reassurances only underline the arbitrariness of its legal interpretation.

Even if the contested terms were ambiguous, the plaintiffs say, the ATF's new interpretation would violate the "rule of lenity," which requires that ambiguous criminal statutes be read narrowly. They also argue that the rule is "arbitrary and capricious," in violation of the Administrative Procedure Act.

A brief filed by the Cato Institute elaborates on that last point by highlighting the result-oriented process that led to the rule, which "was a fait accompli" before the ATF even began to reconsider the issue. "Expressly declining to pursue a legislative solution, [Trump] directed his administration to redefine bump-stock devices…as automatic weapons," the Cato brief says. In response, the ATF "broke from decades of precedent and discovered a new power to prohibit that widely held type of firearm accessory. This expansion of regulatory authority, motivated by political expediency, is arbitrary and capricious."

The ATF's arbitrary and capricious rule unjustly criminalizes peaceful behavior that not only does not violate anyone's rights but does not violate the law by any reasonable interpretation. The DOJ estimates that Americans own as many as 520,000 bump stocks. Since the Supreme Court has declined to intervene by issuing an emergency stay that would have prevented the ATF from enforcing its rule while the case is pending, potentially hundreds of thousands of law-abiding Americans are now exposed to prosecution for possessing products that everyone thought were legal until Donald Trump decided otherwise.