Federal courts have used an extremely broad interpretation of the Commerce clause in other cases. If the iron for the steel wasn’t mined in Montana, it would have to be shipped in, therefore the guns are subject to regulation by the Federal government under the Commerce clause.
There are copper and platinum mined in Montana, but not iron.
Copper is good for the jackets... but you’re right that the steel cores would be difficult to find w/o iron.
But, if shit hits the fan, let’s see how much iron is in those old tractors! ;)