What do the patches look like after firing? They may give you insight.
Sometimes when a 1:48 twist barrel on a rifle shoots conicals well, but not round balls, AND the patches are good, the problem is the factory barrel. The crown can sometimes be "off". For the folks who don't know the term, the edge of the rifled portion where it ends at the muzzle is called the "crown". A lot of originals didn't have a crown on their muzzles with the rifling ending at the edge of the muzzle, BUT factory barrels for muzzleloaders today usually have a slight bevel where the rifling ends at the muzzle. IF that has been damaged over time from the use of cleaning rods, that can be a problem. If it wasn't square with the barrel, when the barrel left the factory that too can be a problem, and a conical will work because of the much longer bearing surface, when a round ball won't.
if you are going crazy trying to get a PB to shoot well and not pleased get a Lee REAL bullet mold melt the balls you have and try the REAL bullet . It shot very good in a .58 cal 1x48 twist Hawken I have. so easy to load
That’s why there were so many replacement barrels made for them. I don’t blame the 1-48 twist as much as the depth of the grooves and wide lands. I don’t have much patience for a picky barrel. They are good deer guns with the LEE Real bullet.
I had a 50 caliber T/C hawken for about 20 years. I never could get a roundball to shoot with any acceptable accuracy. At 50 yards I was lucky to keep the balls in a 3 foot by 3 foot square. I went to Hornady buffalo bore conicals and they were tack drivers. At 50 yards I could keep the shots in a 3 inch circle every time. That hawken was bought in '73 and had a douglas barrel. It just wasn't a good roundball gun. I did carry a few round ball with me incase I needed a second shot at really close range (think 10 feet) to finish off a wounded deer.