I have never used anything but pure lead in a muzzle loader. Is there any rifling marks in that bullet? what thickness patch are you using?
What is your bullet diameter?
Elsewhere I have posted about using hardened lead in a front stuffer. Most of my knowledge about that comes from the founder of Cast Performance Bullet Company, himself a builder of muzzle loading rifles and former serious competitor.
From the deformation of the two pieces of the ball, it is obvious the ball hit something that caused it to shear into two pieces. The two pieces were side by side under the far side skin, therefore the ball probably hit the far side rib. Had the ball broken by hitting the front side rib, the two pieces would not likely have traveled together to the far side, but would have diverged.
I can't identify any marks from rifling, though I've never found any rifling marks on pure lead balls recovered from game. They flatten too much to show any rifling. See the "ball" below, recovered from a small black bear. No way you will find any rifling marks.
The severe flattening of pure lead balls made me question their suitability for elk, where penetration of 30" or more might be required. That lead me to the "Cast Boolits" forum and ultimately to wheel weight lead.
For the elk I shot a 0.490" ball with a 0.015" patch. One piece of the ball recovered from the elk shows the impression of the patch material.
As for accuracy of wheel weight lead balls, see the 50 yard target below. I fired 10 shots of my hunting loads, though only 9 are clearly distinguishable.