as far as I know, the last culling in SA was done by darting from chopper, (normally Scoline in dart, which can and will kill elephant anyway, due to paralizing breathing, thus suffocation). The downed elephant is thenfinished off in the head, mostly with .308 FN rifles 9military hardball, or loaded monometal solids), sometimes with .375 or .458Win.
But, the way I understand 500grs question: for old time culling, where one walked into a herd and shot as many as possible, I would opt for a normal .458 Win on a Mauser type action. I would like ammo loaded with 450gr monometal solids @ 2150ft/sec. The recoil is very low, so a lot of shots can be fired, without having blurred vision. Also, a bullet as mentioned will penetrate deep enough on all elephant sizes, and will have enough frontal diameter if the S%@$t hits the fan to be considered a stopper.