I can see how there is some connection ... for instance, if a State had a company that made guns e.g. pocket pistols, and every State around them banned the transportation of pocket pistols, then that company would be landlocked and unable to engage in interstate commerce, so I think it follows that the federal government is empowered to require that the states allow transportation of pocket pistols ... but the idea that the interstate commerce power embraces a general gun control power seems untenable to me, and for Montana to say that guns made in Montana and kept in Montana are an intrastate affair doesn't seem like nullification to me, it seems like clarification.
There is no question that the government's regulatory power has its limits, for example, it cannot ban the ownership of firearms, or prevent their use for legitimate purposes such as sporting or self-defense, due to the protections offered by the second amendment.
In order for Montana or any other state to even begin to argue that a machine gun built and kept there was not subject to the interstate commerce clause - and it is important to note that the state could only
argue this opinion, never dictate it - it would need to produce and process all of its own raw materials, power, etc etc as well as handle all of its own banking and finance.
Beyond this, there is the fact that American citizens are free to pursue commerce across state lines, and that a state's ability to regulate interstate commerce is at best subject to the will of the federal government.
It may well be unconstitutional for a state to presume to declare that certain goods could not be sold over state lines. Absent the power to make and enforce such a decree, it's unclear what basis Montana would have in the first place for claiming that goods made there were isolated from interstate commerce.
There may very well be a catch-22 at work where our freedom of interstate commerce makes
all commerce into significantly potentially interstate commerce.