Caninethumper,
I took your suggestion and went to Montana's FWP page and looked at the firearm regulations. You restated what I said, there is no caliber limit, specifically. My post is a no spin zone so I will give the complete regulation as can be found on Montana's FWP website.
Firearms
Caliber: There is no caliber limitation during the general big game hunting season (except as specified under shotgun) for the taking of big game animals in Montana. Match the caliber and firearm to the size of the animal hunted.
The use of poisonous, explosive, or deleterious substances on or in any bullet or projectile is prohibited.
Shotgun: Hunters are prohibited from shooting deer or elk with shotguns, except with lead loads of 0 buck or larger, or rifled slugs.
Restrictions for muzzleloader and traditional handgun areas only:
I copied and pasted this directly from their website. As anyone can see, it is a regulation to match the caliber and size of firearm to the animal hunted. While one may choose to argue that there is no minimum caliber I would argue that through ambiguous blanket statements (match caliber to game size)they have set a minimum without specifically saying what it is. This covers all big game without having to list a caliber for each animal. While we can have a lengthy arguement about .223 for deer being big enough most wouldn't claim it is big enough for moose. I believe the FWP has an opinion on minimum caliber for each animal and will act on that opinion.
http://fwp.state.mt.us/ is Montana's FWP website if anyone would like to go look for themselves. Hope this helps. Maybe Mike Granger would know a game warden that would like to weigh in on this.