An illegal combination, under federal law. Regardless of the legality at the state level, marijuana is still classified at the federal level as a schedule 1 prohibited substance. This means that possessing both is a federal crime, punishable by up to $10,000 and as many as ten years in prision.
The Montana Free Press contacted the Bureau of Alcohol,Tobacco and Firearms to confirm, learning that the Federal Gun Control Act prohibits anyone who uses a controlled substance from purchasing firearms or ammunition.
Even if its medical? Yes. Even if it is medical, federal law still prohibits possession.
While the possession of marijuana alone is still a federal crime, it carries only a $1000 and up to a year of jail time for the first conviction. Add a firearm to the mix, and the potential consequence is multiplied by ten.
The state of Montana has an estimated 66% rate of gun ownership. Estimates suggest that about 20% of the adults in the state use marijuana, but those are probably low (given that using was illegal at the time of the survey, it seems very likely that people would under-report). Given these numbers, it is very probable that the two groups intersect. Federal law makes that risky.