I would try some prb and 40-50 grs fffg.
But I think the Lee REAL conical, with an overpowder wad should do well, especially if the riflings aren't deep.
I bet someone on here has a 50 cal REAL mold and would send you few. I have the REAL in 58, 54, and 40 caliber.
I never tried the Lee REAL conical? I shot RBs. ,nothing I tried in bullet form gave me the accuracy I wanted? My Ithaca Hawkin loved every brand of ball I ran through it. I would love to give them a go. I am more curious than anything else. My barrel is .54 Cal. and has a 1 in 66 twist and doesn't care about conicals. What is the weight of a '54 cal. Lee Real bullet?