I have used both 50 cal.,295 gr and 348 gr PB"s, both the aerotip and HP for elk,and have killed more than a few.I used 100 grains of Black Powder.( BTW, remove the plastic tip and the aerotip is the same as the HP)
I have serious doubts that they don't penetrate.I suspect the shooter who did have problems tried to break bone which pure lead bullets have problems doing.In muzzle loader hunting, one must be a little more careful about what shots to take and where to put that bullet in the anamal
Here is 295 gr 50 cal PB I dug out of the offs side of a cow elk I killed.
Here is that cow. The lump on her side is that PB after it transversed her body in a quartering towards me shot.