A federal appeals court rejects a highly implausible redefinition of machine guns.

Two years ago, peaceful, law-abiding gun owners across the country became felons overnight, thanks to the Trump administration's ban on bump stocks. But as the U.S. Court of Appeals for the 6th Circuit recognized last week, Congress alone has the authority to define new federal crimes, while the president and his underlings are charged with enforcing those laws.

Combining both powers in a single branch is a license for tyranny—a danger the Framers tried to avoid by carefully separating the legislative and executive functions. Regardless of their views on gun control, Americans who care about the rule of law should be troubled by the implications of letting unelected bureaucrats unilaterally and arbitrarily criminalize previously legal conduct.

Bump stocks, first patented in 2000, allow rifles to slide backward, propelled by recoil energy, after a round is fired, which resets the trigger. The sliding stock facilitates a rapid firing technique in which the shooter maintains forward pressure on the rifle, causing his stationary finger to bump repeatedly against the trigger.

These accessories were mainly of interest to hobbyists, regulators, and industry insiders until October 1, 2017, when a gunman murdered 60 people in Las Vegas. Because the killer's rifles reportedly were equipped with bump stocks, Donald Trump responded to the massacre with a promise to ban them by administrative fiat.

Tasked with inventing a legal rationale for a ban the president already was determined to impose, the Bureau of Alcohol, Tobacco, and Firearms (ATF) reinterpreted the statutory definition of machine guns to cover bump stocks. But as the ATF itself had repeatedly recognized over the years, that reading of the law was inconsistent with its plain meaning.