Bell used a number of rifles, inc. .256 Mannlicher (6.5mm), .275 Rigby, .318, .450, and maybe, a .416. I don't personally recall him mentioning a .416, but it's been years since I read his books. I need to get one out and re-read it.
He was a superb marksman and he knew elephant anatomy like a surgeon. He found that his light rifles, with good roundnosed FMJ bullets, would kill as well as a .450...for him!
He certainly owned at least one .275, and with it killed more elephant and other animals than with other calibers.
He lived for a few years after the .308 Winchester was intoduced, and commented that he liked its short length. (He disliked the long bolt throw on magnum length actions, for .416, .375 H&H, etc.)
Bell was a remarkable man, and an excellent writer. He must have had nerves of steel.
Lone Star