Remembering the results others folks and I got with patched round balls in 1:48,.54 T/C Renegades 30 years ago, Regulis' experience is pretty typical. The problem is the T/C twist: perfect for conicals, but way too fast for PRB at any meaningful velocity.
With this rifle, my best PRB accuracy at all ranges was with 60 grs. GoEx FFg. Bench rested, this yielded three-round cloverleafs at 50 yards, but by 100 yards these groups had opened to eight inches or more. Patch condition -- marked but not blown -- indicated a good fit. Don't remember the lube, but it was probably some T/C product.
The same rifle with home-cast Maxis and a 100-gr. FFg charge would shoot maybe five-inch 100-yard three-round groups without cleaning between rounds, half that size if I swabbed out the bore after each shot, again with GoEx. The Renegade really came into its own when the 425 gr. Hornady Great Plains bullets became available: with those and the same 100 gr. GoEx charge, swabbing between rounds, it would shoot 100-yard cloverleafs.
For reliable round-ball accuracy with heavy charges, I'd recommend a .54 Green Mountain drop-in barrel, 1:70 twist. With that barrel on the Renegade stock, .535 ball, .015 patch, Dixie Old Zip Patch Grease, again swabbing between rounds, my best accuracy is also the max load, 120 grs. GoEx: benchrested, three rounds reliably in two inches at 100 yards, sometimes an inch and a half, once -- on an absolutely windless day -- a cloverleaf. This load chronographs at about 1900 fps, which yields about 1815 foot pounds or approximately 1100 fps and 610 foot pounds at 100 yards.
Don't think Regulis mentioned what propellent he's using, but that could be another part of the problem. Traditional BP firearms perform horribly with so-called "replica" powders. In fact I don't know what I will do when I finally run out of real black powder. It's gotten impossible to buy locally due to Patriot Act hysteria and local anti-Second Amendment zealotry, and the artificial stuff won't even ignite properly, much less yield reliable accuracy (unless you shoot an in-line gun using shotgun primers, which to me is an unnatural act).
Oh, well; probably doesn't matter anyway: the Democrats took the whole state government this year, and one of their legislative intentions is to outlaw all lead projectiles, which will end muzzle-loading shooting in Washington state forever.