The Bump Stock Ban Highlights the Danger of Letting Bureaucrats Invent Crimes

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...