Sure it's legal in Montana. There's no handgun season if that's what you're asking. There's no weapons restrictions during the general season, so you can use a handgun if you want to. There's a few areas that are restricted weapons hunts [like the area around Bozeman] where you have to use a straight walled cartridge originally designed for use in handguns. Not many people use handguns though. Distant shots are more common than not and people that like restricted weapons hunts use their bows during the archery season.