I've been shooting black powder for the last 15 yrs. And rather your shooting black powder or a cartridge rifle you'll find every barrel is different. Some barrels favor either a lighter grain or heavy grain bullet.
If you reload powder grains and bullets could make a big different on how your rifle shoots. You just have to find the best combo for your rifle. It's almost like each barrel has it's own personality. I use clean shot in the place of pellets in my rifle..I can normally shooot 3-4 times before cleaning, pyrodex and black powder I'd have to clean between every shot. Try T/C sabots and see if that makes a different...I haven't got my 209x50 in yet, but you can bet I'll try around 8 different brands and combos and different powder charges until I get the best groups I can...It's worth the time, money, effort.