After the tragic mass shooting in Las Vegas, the phrase “bump stock” entered the public lexicon. 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 has the ability to write new laws. So the administration attempted to give itself such a power 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. Bump stocks, which require substantial user input to fire, had never been considered “machineguns,” with precedent spanning multiple administrations. 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 not limited to a ban on bump stocks. ATF has asserted the complete authority to ban any new class of weapons that federal law did not address. This approach impermissibly expands the executive branch’s power to rewrite criminal laws and threatens to stifle new developments in firearm technology.

The new rule, making felons of an unknowable number of Americans, was set to take effect March 21. To prevent this, a group of Second Amendment organizations filed a lawsuit in federal court. They sought a preliminary injunction to stop the government from enforcing the new rule, but were denied. Because the effective date is so close, the appeal to the U.S. Court of Appeals for the D.C. Circuit was expedited. Cato filed a brief addressing 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 write new ones. The administration argues, essentially, that because the statute did not provide a separate definition of the terms that make up the definition of “machinegun,” that it gets to insert their own meaning. 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 bureaucratic reimagination.

The D.C. Circuit hears argument in Guedes v. BATFE on March 22.