I know that there are VERY difficult to find games about this era. Supposingly that Old West are Victorian too, here what I found. But lets share what do you got.
Age of Rifles
Old hexagonal wargame, wich I havent played. But It have Boxer rebellion!
Victoria: An Empire Under the Sun
Global strategy game
Mount and Blade - 1866 Western Mod
Mod for M&B with many muskets and single shots rifles! I love when you play as US to train many single shot rifles infantry and fight Indians near villeges (there will be no horses in villages) or storm theit forts. Why? Because your army will face a horde of spear and bow. And battle will be like in ZULU! I've even putted Zulu Soundtrack into mod for better experience! Very good mod, where you can imagine what it was like to shoot Martini-Henry (Mod is still in development)