I think three things are in play here...
1. No one wants to be the guinea pig and build a machine gun within Montana and risk becoming a felon and all that comes along with it.
2. Unless you can prove that EVERY part and all the metal/wood/polymer that went into the firearm was not produced, created, built, machined or mined in another state, the Feds will find a way to bring it into the fold of interstate commerce.
3. Wickard v. Filburn - SCOTUS case and the ruling on what is defined as interstate commerce.