I am no lawyer but there are a couple of clauses from the Montana Firearms Freedom Act and how I interpret them.

Section 3 states

(4) “Manufactured” means that a firearm, a firearm accessory, or ammunition has been created from basic materials for functional usefulness, including but not limited to forging, casting, machining, or other processes for working materials.

So unfortunately it looks like 90 / 80 / 70% finished receivers imported into Montana and completed will not be considered “Made in Montana”.

This also means Montanans will not be allowed to chop down their rifle barrel or add a foregrip to their handgun without running a foul of the BATFE, unless the gun was made in Montana.

Section 5 (restrictions) states:

(2) a firearm that has a bore diameter greater than 1 1/2 inches and that uses smokeless powder, not black powder, as a propellant;

What this means is that while 20mm rifles/cannons are not going to be classified as destructive devices if made in Montana, and we may even seen 30mm cannons manufactured there. That will be exciting!

1.5″ is equal to 38.1 millimeters so unfortunately 40mm RPGs will not be legal due to the caliber restriction 🙁

A big thanks to Jay for the info about the Act.