In answer to the original question, if KDM Bell didn't need anything more than a 6.5 or 7mm to kill over a thousand elephants, then I guess I'll be okay shooting NA game with 6mm, .223, or 6.5 bullets that are leaps and bounds superior to what KDM Bell had, not to mention a much better rifle, optics, ballistic computer and a range finder.
en.wikipedia.org
Interestingly, Bell also appreciated the 22 Savage High Power for Red Stag and buffalo, and some used it for tigers. Again, with inferior bullets, rifles, optics, and no ballistic computer or range finder.
en.wikipedia.org