The Supreme Court overturned the Court of Appeals' decision in Staples v. United States, ruling that the government must prove beyond a reasonable doubt that the defendant knew the weapon they possessed met the statutory definition of a machine gun to convict them under the National Firearms Act. The petitioner was charged with unlawful possession of an unregistered machine gun, but claimed he was unaware of the automatic firing capability of his unregistered AR-15. The District Court rejected his proposed instruction and charged the jury that the government need only prove that the defendant knew he was dealing with a dangerous device of a type that would alert one to the likelihood of regulation. The Supreme Court granted certiorari to resolve a conflict in the Courts of Appeals concerning the mens rea required under § 5861(d). The legal principle of mens rea is generally required for criminal offenses, but Congress may impose strict criminal liability for public welfare or regulatory offenses. The government argues that this case falls under this category and does not require proof of mens rea. However, previous cases have shown that mens rea is required unless criminalizing innocent conduct. The court has recognized public welfare offenses in limited circumstances, but guns cannot be compared to hand grenades, and there is a long tradition of lawful gun ownership in the US.

The court ruled that mens rea is required for offenses punishable by imprisonment, and the usual presumption that a defendant must know the facts that make his conduct illegal should apply. The government's argument that guns should be exempt from mens rea is invalid. The issue is the level of knowledge required to convict under § 5861(d). The absence of a knowledge requirement in § 5861(d) suggests that Congress did not intend to require proof that the defendant knew all the facts that made their conduct illegal. The National Firearms Act is a public welfare statute designed to regulate the commerce and possession of certain dangerous devices, including specific types of weapons, to protect public health and welfare. The court has read a knowledge requirement into public welfare crimes, but not a requirement that the defendant knows all the facts that make their conduct illegal.

The petitioner knowingly possessed a semiautomatic weapon that could be converted into a machinegun, which is regulated by the National Firearms Act. Mens rea must be applied to the statute to avoid punishing innocent conduct. The government argues that a conviction can be supported by proof that the defendant knew the item was highly dangerous and likely to be regulated. The author disagrees with Justice Ginsburg's approach to some scenarios and supports the conviction as the defendant knowingly possessed a dangerous device that would alert one to the likelihood of regulation.