Depending upon the actual ball and powder charge your 10 ought to be "quite adequate"

Figure that in the old days the dangerous game loads for a 10 bore (cartridge gun) would have been 8 Drams (218 grains) and a round ball or conical. Hardened ball usually cast of 1:15 mix & maybe added mercury. ……sorry no mercury these days.

Just be aware that you cannot easily (as in basically impossible) ship Black Powder for your cartridge gun or muzzle loader to any of the places where you might hunt Cape Buffalo. So you "may" be limited to whatever locally sourced & problematic "black" is available in country.

According to the vintage reports, and a few modern reports, the 10 and 8 bores are quite adequate for making the Cape Buff "Hors De Combat".
- Mike