Take a 50 BMG API round, turn on a lathe down to .500 and shoot it through a S&W 500 (or 50 cal muzzleloader for that matter)?
Taking down from .510\.511 to .500 may negate crimping on the cannelure. Bullet would probably end up around 600gr. Would be interesting to see what it does on elephant and buff.
Morbid curiousity drives me to ask the question.

Such a long, slow bullet would be very difficult to stabilize and even if stabilized in flight would probably tumble upon impact, resulting in shallow penetration on such large animals.

Neither of the guns you mention have any chance of stabilizing a bullet that long. Look at the twist rate for a 50 BMG and try to get the same rpm at the velocity you can get from your gun.