My vote would be for a 45-70 also, have an 1886 Browning, 1885 Winchester and Shiloh 1874 in that caliber, I like KISS. If you wanted something different that is easy to form available brass for and much lower recoil than the 45 calibers, how about a 40-60 Maynard. Form brass from 303 British or 30-40 Krag. Enough power to knock down the rams if you get fast enough twist to handle 400 grain boolits if that is a consideration.