Good Afternoon:
In the 70's Navy Arms had a ad. for their magnum type muzzleloaders in 58 cal. for the Big 5 in Africa. It was used successfully on all of the big 5. They gave a load of 175 gr. of FFF powder and a 600 gr. projectile for a muzzle energy equal to the 458 win. mag.. I am not giving a personal recommendation for this load but Navy Arms did, I bought one. I have only shot 150 gr. of FFF with a 505 gr. bullet,,,, it was a really stout recoil, I have not shot that load again!!! I named the rifle Buffalo Gal.
I think they are still available for about 700.00 IIRC from Navy Arms or maybe Dixie Arms.
safeyt first
HM