Well, according to the Hodgdon Data Manual #26, page 743, Pyrodex RS loads for the .58/.577cal rifle are

315gr Minié bullet - 75gr

505gr Minié bullet - 70gr

Using the BP/Pyrodex equivalence, this equates to a 2Fg BP load of 79 and 73.5gr respectively. As I mentioned before, the SERVICE load, as used by the British Military from the late 1850's until the advent of the Snider conversion, and both sides during the WoNA, was only 68.5gr of what they then called 'fine rifle powder', so my advice to start off VERY low still holds.

The service load with the British 535gr Minié bullet is not a load of fun, if you'll forgive the slight play on words.

tac