Sarge:
I would recommend hardened lead for buffalo. Sooner or later, the time will come when you need to break the on-side shoulder and still penetrate the chest cavity. Soft lead may work, but that is fairly high velocity for un-hardened lead and the possibility of failure by over-expansion on the big bones cannot be dismissed.

Too hard, though, and the ball may break up. Read Baker for the best info on this topic IMHO, but try to avoid hardening with mercury!

Others here will have first-hand experience I'm sure, or be in a position to recommend the best alloy for high velocity RBs on big game.

Taking that Tolley after buffalo would be a hoot!