American , Huh ?
Buffalo...okay , let's see.
I would have to recommend ( under the circumstances since you specified American ) a .458 Winchester Magnum loaded with 480 gr bullets. I know that the smart , modern thing to do is to use expanding bullets , but l can Personally attest that Monolithic meplat brass Solids are good too for Buffalo ( assuming the Australian Water Buffalo which l have shot is similar to an African Cape Buffalo ) , provided you don't have to worry about over penetration ( just make sure no other animals are behind your buffalo ) . Also never try loading a .458 Win Mag with 500 gr Monolithic solids . I have personally seen my Handloading buddy load them into the cases and l was appalled by how much the ( already marginal ) case capacity is reduced. Use 450 - 480 gr Monolithic solids or expanding bullets like Swift A frames .
I would recommend the .458 Lott over the .458 Win Mag , but then , you specified American , and Jacques Lott was French