The cast bullet aspect is going to be the clincher in your selection. Check out Rotometals for good casting alloy and “White Label” lubes. I like their Carnuba Red which they say is good for 2700+ fps.
You would not go very far wrong with a 45-70. You would have to consider whether to go single shot or lever gun, though I have a friend who built a Chinese Mauser into a workable 45-70. I notice you specified one caliber and not one gun. You could always do a lever gun for the up close work and...
Retired to the hills of Tennessee and having a good time getting back to shooting my sporting guns rather that my working guns. I shot service rifle and long range matches off and on for a few years out on the west coast but family demands kept things simmering. Bob Sutton (now deceased) built...