It looks like they're trying to explicitly define firearms "manufactured and retained in Montana" as outside the jurisdiction of the federal government, being specifically intrastate, rather than interstate commerce. This would, in theory, invalidate federal gun laws for such firearms, at least until such time as a federal anything looks at it, which, according to section 7, would be as soon as someone in Montana decides to manufacture a firearm. It specifically doesn't apply to full-auto stuff (section 5).
The only real uses would, I think, be suppressors manufactured in Montana and kept in Montana, and it would work against any federal "assault weapon" ban that might come up in the next few years, since the banned firearms would not fire "two or more projectiles with one activation of the trigger."
Good luck with that.
AJ Dual posted while I was typing, and I agree with him, that it's designed to force a test case for an interstate commerce fight, for a firearm which has never crossed state lines.