Staples (defendant) possessed a semi-automatic rifle that originally had a metal piece preventing it from firing automatically. Staples filed down the metal piece. As a result, the rifle met the statutory definition of a firearm under the National Firearms Act, 26 U.S.C. § 5861(d). Staples did not register the weapon as required by the act. The United States (plaintiff) charged Staples under the act, which makes possession of an unregistered firearm punishable by up to ten years in prison. Staples claimed he did not know the rifle could be fired automatically. The trial judge refused to give Staples’s requested jury instruction, which stated that the government was required to prove that Staples was aware that the gun would fire automatically. Instead, the judge instructed the jury that to sustain a conviction Staples only needed to know that he had a dangerous device that should have alerted him to the possibility of regulation. Staples was convicted, and the court of appeals affirmed. The United States Supreme Court granted certiorari.