I shot one shot with a .600 NE into a downed but not dead hippo bull. Recoil not painful, but it really pushes you too far back. (Spanish clients rifle.) This client has shot 4 elephant and one hippo (above one) with this rifle in my presence, and though the penetration impressed me (unlike what you read about it), the overall performance was nothing more than a good .458... Propably due to not so good shot placement. What was interesting is that shots into the face of bull elephants 12 yards and 25 yards away did not "knock them down". The one did stumble, but both times the bullet missed the brain, and the elephant could have made an escape if the client's brother and himself did not pour some more lead into it in very short time. (On the first instance related here, the brother was using a .500 Schuler bolt actioned gun, second time a .577 NE double.) On the penetration- my shot into the brisket of the downed/ stunned hippo, exited between his shoulder blades, after smashing through his spine. On most of the elephant, it was difficult to try and work out which bullet went where, but the skinners have recovered 2 so far (he is hunting with me again this year), one in the head, and one in the paunch.
The only ammo seen used so far- Wolfgang Romey factory loaded solids (Woodleigh's).