One thing i've discovered about shallow groove 1-48 twist barrels. You can't push the PRB too hard. I read it can skip over the rifling if you do. I'm not sure that's what is happening. I do know I can't get good groups if I load too much powder.

My Renegade with the stock barrel likes 75-80 gr of BP 2F, If I go any higher with the powder the groups fall apart. I can push the deep groove GM barrel as much as my shoulder can handle, and it still groups ok.

With all that said. Have you tried lighter loads?

Will a 75 gr PRB load kill an elk? I don't know, but i'm going to find out this fall. I was going to try the maxi ball, but the recoil is no fun, and it started to get to me about not being in the spirit of traditional hunting. It was fun messing with it though. If you think a sore shoulder /cheek is fun.