I am familiar with most of what you said and, for the most part, agree. But you also said:
. Where did that come from? A barrel is a barrel and is/was made as the maker made it. Statistically, I doubt that is correct. Thee are a lot of .58 Hawkens, Jaegers, bench rest and other style rifles in that caliber which are no make over military muskets. Nowhere did the OP say his was military. What is missing from this discussion is his twist rate. That factor is of major consideration in finding the sweet spot load. Until we know that we cannot offer him more definitive assistance.A lot of the .58's, in fact, most of them have barrels made for shooting the .58 Minie' balls.