Karamojo Bell is well documented in having taken hundreds of elephant with a 7x57mm Mauser. The big bullets are necessary for stopping a charge, but any bullet that you can place into the vitals will do the job, be it the brain, spine or heart/lungs... and that's just a matter of understanding what bullet channels are available in the animal and having the opportunity to place your shot.
And I'm not suggesting that Mr. Bell was a poacher.
Heck, here in the U.S. I am pretty confident that the .22LR is the biggest killer. Poachers in Canada take moose with them. Just keep peppering the lungs until the animal can't take a breath and collapses. Up until that point the damned thing just thinks it's getting bit by mosquitoes!