But the ATF already has “drawn a bright line.” To be more precise, it has applied the bright line drawn by Congress in the National Firearms Act, which defines a “machinegun” as a weapon that fires more than once “by a single function of the trigger.” A rifle equipped with a bump stock does not fit that definition, since it still fires just once per trigger pull.

The ATF has repeatedly affirmed the legality of bump stocks—in a 2010 letter to Slide Fire Solutions, which makes one version; a 2012 letter to a competing company, Bump Fire Systems; and a 2013 response to an inquiry from Rep. Ed Perlmutter (D-Colo.). As the agency explained to Perlmutter, “Bump-fire stocks (such as the Slide Fire Solutions stock) that ATF determined to be unable to convert a weapon to shoot automatically were not classified as machineguns.”

Asking the ATF to revisit this question means asking it to ignore the law. Last week House Speaker Paul Ryan (R-Wis.), following the NRA’s lead, claimed “the regulatory fix is the smartest, quickest fix.” But as Feinstein noted, “The ATF lacks authority under the law to ban bump-fire stocks.”