On my one and only trip to Africa I used a .264 with 140 grain X bullets to take 3 Gemsbok, 1 Zebra, Kudu, Leopard, and a Hartebeast. Only 1 Gemsbok needed a 2nd shot.

This last season my son and I took 4 elk with a .260 and a 6.5x284. We used nosler 140's. All went down with one shot.

I would listen to the folks with much more african experience than I on what rounds to use but the 6.5 will do the job if you place your shots well.