Before the tragic mass shooting in Las Vegas, almost nobody in the United States had ever heard of a “bump stock.” What was, and always has been, a gun-range novelty was suddenly the subject of national discussion. In the months following the tragedy, Congress considered and ultimately rejected a law banning these devices. Eager to seize political capital, however, the Trump administration sought to ban them anyway.

The administration faced one problem, though: the Constitution. As anyone who’s seen School House Rock can tell you, only Congress can write new laws. Never to let something like a written constitution get in their way, the administration tried to make new law by “reinterpreting” an existing law: the National Firearms Act of 1934 (NFA), which heavily regulates “machineguns.”

For decades, Congress, the executive branch, and the people shared a common understanding: the definition of “machinegun” in the NFA was clear, applying only to weapons that fired continuously from a single function. Be it with a button, a lever, or a traditional trigger, a “machinegun” fires continuously upon the performance of a single function. Bump stocks, which require substantial and continuous user input to fire, had never been considered “machineguns.” President Trump announced that his administration was changing course. The president expressly declined to go through Congress, instead directing officials to redefine bump-stock devices as “machineguns.” In turn, the Bureau of Alcohol, Tobacco, and Firearms (ATF) broke from decades of precedent and granted itself a new power to ban a widely owned firearm accessory.

This expansion of regulatory authority, motivated by political expediency, cannot stand. Whether one agrees that bump stocks should be regulated or not, this change is about far more than bump stocks. ATF has asserted the complete authority to ban any new class of weapons that were never covered by the 1934 law. This approach impermissibly expands the executive branch’s power to rewrite criminal laws and the casual approach to ignoring the Constitution would certainly not stop with the ATF if allowed to stand.

The new rule, making felons of an unknowable number of Americans, took effect on March 26, 2019. Gun owners and advocacy groups filed lawsuits in several federal districts, including one that ended up in the U.S. Court of Appeals for D.C. Circuit in which Cato also filed.

Another case is now before the U.S. Court of Appeals for the Tenth Circuit. It was brought by the New Civil Liberties Alliance on behalf of Clark Aposhian, who lawfully purchased a bump stock. Our brief here addresses issues that no other amicus discusses: that the executive branch cannot use the administrative process to accomplish legislative goals that Congress declined to enact.

The implications of this case extend far beyond bump stocks. Regardless of what public opinion is at this moment, the law means what it says. The executive branch has the power to interpret existing law, not create new ones. The administration argues, essentially, that the clear political motive here doesn’t matter, and that nothing prevents them from inventing their own definitions of the terms that define a “machinegun.” That simply isn’t the case. Administrative interpretations are supposed to do just that—interpret existing law—not give new meaning to an old one.

If the government really wants to regulate bump stocks, it needs to do so by passing a new law, not by assigning new meaning to an old one. The Founders weren’t short-sighted; there’s a reason laws that affect the entire nation have to come through Congress, not through politically motivated bureaucratic reimagination.

The Tenth Circuit will hear argument in Aposhian v. Barr this summer.