If it involves inter-state sales and is thus under Federal jurisdiction, if not then State law might trump Federal law/the Feds might not have the authority to enforce the law. Though that too might just have been a result of certain loopholes? (For instance, previously -this might have changed after Obama's new executive order- Federal law prohibited felons from carrying guns, while certain states allowed felons to do so often after a certain amount of time. As long as those felons remained within the state, they weren't breaking the law.) Perhaps it depends on who the agent who sees the gun works for, but I always get confused when a Federal agent has the jurisdiction to act, technically only if something involves multiple states, right?