Suppose someone smokes pot and fills out a 4473 saying they don't. It's illegal to lie on a 4473 per 18 USC 922(a)(6). Here is the big, SO WHAT, if someone smokes pot, can they lawfully purchase and lawfully possess? I don't know. Interesting read from Wikipedia - Haynes v. United States. The National Firearms Act was amended after Haynes to make it apply only to those who could lawfully possess a firearm. The National Firearms Act of 1934 required the registration of certain types of firearms. Miles Edward Haynes was a convicted felon who was charged with failing to register a firearm under the Act. Haynes argued that, because he was a convicted felon and thus prohibited from owning a firearm, requiring him to register was essentially requiring him to make an open admission to the government that he was in violation of the law, which was thus a violation of his right not to incriminate himself. In a 7-1 decision, the Court ruled in 1968 in favor of Haynes. Earl Warren dissented in a one sentence opinion and Thurgood Marshall did not participate in the ruling. This eliminated prosecution of prohibited persons, such as criminals, and cured the self-incrimination problem.