Tens of thousands of American bison have been taken with some flavor or another of black powder, cast .458/.459 (45-70, 45-90, 45-110). The ideal conditions for a .45-70 are little different than the ideal conditions for an un-scoped double, are they not? Would you take a 200-250 yd shot over iron sights with a .577 NE at a cape buffalo? I can shoot clover-leaf with my .308 all day long from 200 yards, but I'm probably not going to ever pull the trigger on a cape buffalo, even with a scoped 375 or 416, from 200-250 yards. Some guys would, I just wouldn't.
After reading Vince Lupo's accounting of his safaris (
http://www.garrettcartridges.com/luposafaris.html) with a .45-70, I'm as convinced as anyone that a .45-70, with the right ammo, is more than up to the task. Lupo (or Garret Cartridges) may be completely F.O.S. for all I know, but he claims full body-length penetration of hard-cast, 540 gr Hammerheads as well as a broad-side shot from 80 yards which broke both shoulders on an old dagga boy with a 13" boss. From within 100 yards, the NE and magnum cartridges can do no better (is there a "better" than 2 broken shoulders and full body length penetration?).
I've already found a PH in Zim who is fine with me bringing a .45-70 for buffalo, and I'm sure he isn't the only one. Now, having said that, my Marlin is going to get an upgrade from either Ranger Point Precision or Brockman's before I go.
I hope this doesn't come across as snarky or anything, and if it does, you have my apologies ahead of time.