The federal government (plaintiff) prosecuted Ernest Camp (defendant) for possessing a machine gun, in violation of 18 U.S.C. §§ 922(o)(1) and 924(a)(2). Section 5845(b) of Title 26 U.S.C., a statute defining various types of weapon, described a machine gun as "any weapon which shoots . . . automatically more than one shot, without manual reloading, by a single function of the trigger." Section 5845(b) also used, but did not define, the term "trigger" in describing shotguns and rifles. The trial evidence established that, pursuant to a warrant, police searched Camp's home and found illegal drugs, drug-manufacturing equipment, and weapons. One of the weapons was a semiautomatic rifle that Camp had modified by installing an electrical switch, a small motor, and a fishing reel. When flipped on, the switch activated the motor, which spun the reel. With this contraption, Camp could fire multiple rounds in rapid succession for as long as he squeezed the rifle's original trigger. A federal Bureau of Alcohol, Tobacco, and Firearms (ATF) agent testified that Camp's electrical switch was not a legal "trigger activator," because the ATF defined a trigger activator as a spring that merely facilitates pulling a gun's trigger to squeeze off one shot at a time. Nevertheless, the trial judge accepted Camp's argument that the switch was not a "trigger" within the meaning of § 5845(b). The judge granted Camp's Federal Rule of Criminal Procedure 12 motion to dismiss the case. The government appealed to the Fifth Circuit Court of Appeals.