I wouldn't used a 45cal PRB on a deer beyond about 75 yards. A buffalo (bison)? forget it!
What killed most of the bison were cartridge firing rifles from the 50-70 in the early years to the 45-100 in the later years. The 50-70 with a 450gr bullet was pretty effective. The 45-70 didn't appear until 1873, and larger 45-90, 45-100, and 50-90 were used later. The 44-77 was also a relatively early and powerful cartridge.
The 50-70 was first available in converted Sharps and Trapdoor rifles that had originally been percussion ignition. Surplus military ammo for cheap made these popular.
What most of these cartridges had in common was a long, heavy bullet that would penetrate. A PRB doesn't have this kind of penetration, has a low BC so loses velocity quickly, and is a 100 yard projectile at best. While 20 bore (62cal) or larger PRB might have a chance at killing bison, the guns made for such primarily were Jaegers(Germany/Austria/Switzerland) or English sporting rifles. The Jaegers preceeded the "Kentucky rifle" here in North Amercia, so large bores were almost non-existant (here)by the mid 19th century. The English sporting rifles were used successfully in Africa, but large bores(up to 4 bore or so, meaning 4 RBs to the #) and heavy rifles were still short range weapons.
