If you like rifles as well as shotguns, stop in Big Timber and visit C. Sharps Arms and Shiloh; they're right across the street from one another. Call ahead and they'll give you a tour. (Leave your wallet with a trusted third party.....). Take just enough $ to pick up some fancy bits of walnut from their scrap pile!
If you go to the Custer Battlefield, take the time to see the Benteen and Reno sites as well. People tend to forget about the soldiers that fought and won that day (in that situation, I'd say surviving was winning enough!). And remember you have to think of the land as it was then, not the cleaned up landscape that's there now. Fighting in dense brush is even less fun when you have an enemy who just LIVES for "up close and personal"!