After 2017's mass shooting in Las Vegas, Donald Trump vowed to use the powers of the presidency to ban bump stocks, a type of firearm accessory that the shooter reportedly used. "We can do that with an executive order," Trump declared. "I'm going to write the bump stock; essentially, write it out….They're working on it right now, the lawyers."

What the lawyers at the Department of Justice ultimately came up with was a new rule amending "the Bureau of Alcohol, Tobacco, Firearms and Explosives regulations to clarify that [bump-stock-type devices] are 'machineguns' as defined by the National Firearms Act of 1934 and the Gun Control Act of 1968" because "such devices allow a shooter of a semiautomatic firearm to initiate a continuous firing cycle with a single pull of the trigger." The federal ban on machine guns, in other words, would now be interpreted by the Trump administration to cover bump stocks too.

"Where was Congress in all of this?" you might ask. "Isn't it the job of the legislative branch—not the executive—to change the meaning of a federal law?" Not according to Trump's Department of Justice. In the final bump stock rule published in the Federal Register, the agency justified its actions by invoking a controversial Supreme Court opinion that says the executive should enjoy broad deference when interpreting the meaning of "ambiguous" federal legislation. "When a court is called upon to review an agency's construction of the statute it administers," the bump stock rule states, "the court looks to the framework set forth in Chevron U.S.A., Inc. v. Natural Resources Defense Council, Inc."

Today, Justice Neil Gorsuch threw a little shade at the Trump administration for unilaterally rewriting federal gun laws. "The agency used to tell everyone that bump stocks don't qualify as 'machineguns.' Now it says the opposite. The law hasn't changed, only an agency's interpretation of it," Gorsuch wrote. "How, in all of this, can ordinary citizens be expected to keep up—required not only to conform their conduct to the fairest reading of the law they might expect from a neutral judge, but forced to guess whether the statute will be declared ambiguous….And why should courts, charged with the independent and neutral interpretation of the laws Congress has enacted, defer to such bureaucratic pirouetting?"

Gorsuch's statement came attached to the Supreme Court's denial of certiorari in Guedes v. Bureau of Alcohol, Tobacco, Firearms and Explosives. Damien Guedes, who challenged the legality of Trump's bump stock ban, recently lost before the U.S. Court of Appeals for the District of Columbia Circuit, which said the ban was entitled to judicial deference under the Chevron precedent. The Supreme Court today declined to hear Guedes' case.

Gorsuch agreed with that. "Other courts of appeals are actively considering challenges to the same regulation," he wrote, and "before deciding whether to weigh in, we would benefit from hearing their considered judgments." But, he added, waiting for the right case to come along "should not be mistaken for lack of concern."

In fact, Gorsuch suggested, the right case could not come along fast enough. "The law before us carries the possibility of criminal sanctions," Gorsuch wrote. "Before courts may send people to prison, we owe them an independent determination that the law actually forbids their conduct. A 'reasonable' prosecutor's say-so is cold comfort in comparison."