The Interstate Commerce Clause effectively means all economic activity in the US is under Federal jurisdiction because even something that's not directly involved in interstate commerce, even something not involved in commerce at all, can have an indirect effect on interstate commerce. In Wilkard v. Filburn the government successfully argued that Federal limits on wheat production were enforceable on a farmer that grew his own wheat to feed his own animals even though the farmer never sold his wheat to anyone and the wheat never left the state. A similar more recent case, Gonzales v. Raich, confirmed that this same principle applied to someone growing medical marijuana for personal consumption in a state where medical marijuana was legal.
In your example, the felon is buying a gun manufactured in the same state. While this doesn't have a direct immediate effect on interstate commerce, its indirect effects are more obvious than in the two cases mentioned above. If it were legal for felons to buy guns made in state, but not out-of-state, then it would have a fairly dramatic effect on interstate commerce. Gun manufacturers would set up local manufacturing operations in many states to make guns for the felon market. (In theory at least, in practice I think most if not all states also ban felons from owning guns.)
Also since guns are durable items, unlike wheat and marijuana, it's all but impossible to show that the gun will never leave the state and participate in interstate commerce directly.