IIRC, the argument over marijuana was that local producers were putting their product on the national market effecting interstate commerce; the federal government recognized no legitimate purpose for marijuana, so making it cheaper to buy marijuana inherently served an aim in opposition to the federal regulation of the “commerce.”
In the gun case, Montana seems to have anticipated this issue. By certifying that they are produced and sold locally, takes them out of the sphere of federal interest. The only way this could be dragged back into a commerce-clause situation would seem to me to be if the federal government affirmed that, as in the case of marijuana, they had a federal interest in reducing demand by artifically, purposely inflating prices, an argument which would put them into direct opposition with the 2nd amendment.