Wait a minute, wait a minute, wait a minute. Misprision of a felony is not a mandatory crime reporting law. Under federal law, misprision not only requires actual knowledge of the crime, but you have to also take some action to conceal the criminal conduct (e.g. lie to the feds). In the OP's example, or any gun range empirically gathered evidence example, you cannot possibly have actual knowledge of an "SBR crime". Furthermore, a person is only charged with knowing the law to extent they must comply with the law - you are not charged with knowing whether or not someone else is complying with the law. Unlike complying with the laws (i.e. ignorance is not an excuse), you are not required to know the law in the imprision context. In other words, ignorance absolutely can be an excuse as to whether someone else is committing a felony. The common people are not law enforcement or judiciary agents. Minding your own business is not a crime unless you are a person who, by law, is under a heightened standard (e.g. child abuse reporting statutes for health professionals). Misprision is essentially an aiding and abetting or obstruction related charge, and nothing more. If you could go to prison for simply being in the vicinity of someone feloniously violating any weapons related laws, then no rational person would go to any gun range or be around anyone who has a weapon unless they are 1,000% vetted by you.
I concur with the "mind your own business" crowd. The OP is presuming that the guy is in violation of the law based on some empirical evidence and what the guy said. Who really knows if he was legally compliant or not other than the feds. If the OP is really worried about it, then file a complaint with the authorities. If the OP is not, then move on (i.e. what's the point of this discussion?).