The National Firearms Act imposes strict registration requirements on statutorily defined firearms, including machineguns. A machinegun is defined as any weapon which shoots, or can be readily restored to shoot, automatically more than one shot, without manual reloading, by a single function of the trigger. In 1986, Congress amended the Act to criminalize the mere possession of an unregistered machinegun, a violation punishable by up to ten years' imprisonment.
In 1989, local police and Bureau of Alcohol, Tobacco and Firearms agents executed a search warrant at petitioner Steven Staples' home and recovered an AR-15 rifle. The AR-15 is the civilian version of the military's M-16 rifle and is manufactured with a metal stop on its receiver to prevent conversion to automatic fire. On Staples' rifle, however, the stop had been filed away, and the weapon had been assembled with an M-16 selector switch and several other M-16 internal parts, including a hammer, disconnector, and trigger. BATF agents tested the rifle and confirmed that it fired more than one shot with a single pull of the trigger. It was undisputed that the weapon was not registered as required by the statute.
Staples maintained that the rifle had never been modified to fire automatically and that he had no knowledge that it was capable of doing so. At trial, he requested the District Court to instruct the jury that the Government must prove beyond a reasonable doubt that he knew the gun would fire fully automatically. The District Court rejected that request.
Instead, it charged the jury that the Government need only prove the defendant knew he was dealing with a dangerous device of a type as would alert one to the likelihood of regulation. The jury returned a verdict of guilty, and the court sentenced Staples to five years' probation and a $5,000 fine. The Court of Appeals for the Tenth Circuit affirmed the conviction.
The Supreme Court granted certiorari to resolve a conflict in the Courts of Appeals concerning the mens rea requirement under the statute.