There was a case years ago in which a Fed ruling was that even if a crop like corn were grown in a state, and it was intended to be sold and consumed in the state, that it could still be transported across state lines, therefore the crop was covered by the commerce clause. I have no doubt with these present courts that some Fed judge will come up with the same reasoning about a gun made in the State of Montana.
A previous poster mentioned the Supreme Court case “Wickard v. Filburn” under FDR. The Court found against a farmer growing wheat on his own land to feed his own cattle! Here’s a link :
http://www.conservapedia.com/Wickard_v._Filburn