The 45-70 and its variants killed most of the buffalo. 45-70, 45-90,45-100, 45-110 and the 45-120 are all good. The 50s came out at the very end of the shooting, the early 1880s, and didn’t play that much of a role.
45-70 is the proper size for a buffalo gun, but I would get a Past shoulder pad, (Magnum) as the recoil can really damage (yes, damage) your shoulder. These guns are very near artillery.
My favorite buffalo rifle is my H&R Buffalo Classic. Based on a very early design (1860s) it looks like an English sporting rifle with a break open action.
It is a real screamer.......
I guess you won't be asking to shoot my 5 1/2 pound, fiberglass stocked, 16.5 inch barrel, .45-70 Guide Gun, eh?
Unfortunately, you would have to dive to see it. It was lost in a tragic ice fishing mishap.