(Note I "pruned" a couple identical posts by Marrakai)

Bell recommended in one of his books a .318 Bolt and a .450 Double as the ideal two rifle battery for Africa. He describes in one safari how his mate used these two calibres while he elected to use his .318 for his elephant hunting.

I remember reading he choose to use a .275 Rigby as his elephant rifle because
* he was hunting in very high grass in the Karamojo region;
* he had to take a lot of shots sitting on the shoulders of a black or a tripod - larger calibres knocked him off;
* the elephants were unused to gunfire and the quieter report did not disturb them as much;
* he could get solid ammo for the .275.

Marrakai

My guess he used a .450 as this was the calibre he recommended.