The earsplitting, heart-stopping roar of fully automatic weapons hasn’t been often heard on America’s streets since Congress largely outlawed them in 1934. But now it’s back, owing to a small device that is easily plugged into certain handguns and rifles, converting semiautomatic firearms into guns capable of firing 20 bullets in one second — with one pull of the trigger. In Sacramento last year, a handgun converted into a machine gun was used during a gun battle that left six dead, in what has been called Sacramento’s deadliest mass shooting. In Minneapolis, eight people were wounded in August amid a...