I use my .270 and 130gn Speer Grandslams for everything I shoot in Africa - unless you're hunting dangerous stuff like hippo, buffalo etc, you'll be fine for everything up to and including Eland. Same applies to 7x57, .260R, .270WSM etc - I've seen them all used with aplomb and animals all dead on the spot.
Most of the plains game species are pretty thin skinned - just buy yourself a pocket copy of The Perfect Shot and make sure you are familier with the slightly different heart/lung placement than found in European large game species. Many PHs like you to use .30s and Magnums because they have a bit more margin for error but if you know your shot placements and are confident at longer ranges off sticks, they're not essential.
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