I brought my Marlin 1895 Guide Gun .45/70 in Namibia and SAR on two occasions and had a chance to take Buffalo cow, zebra, Red H, Blue W, pigs, and oryx with it. I used Speer 400grs FP and 420grs HC WFNGC both on 1,700fps. Soft point was a bit too soft for plaint game but hardcast worked perfectly. For buffalo I used my own construction (something like combination of Punch Bullets made of 100% copper with driving bands) 400grs WFN 1.850fps with extraordinary results; I took a frontal shot at 50m and bullet stopped just under the skin next to the tail. Two side shots made exit wounds so my conclusion is that you can 100% rely on your .45/70 and have a great and successful hunt!
