Some years ago T/C made barrels specifically for round ball use and conical use instead of one-twist-fits-all. Its worth checking the twist.
Check the fit up of the barrel into the patent breech and the stock. If it's possible to wiggle anything more than a very little bit I'd cure that problem before I went to the range again.
Then try going back to basics:
1. Insure a reasonably clean bore.
2. Use a reasonably consistent powder charge. You might also try a different lot or brand of propellent. Most rifles do well reasonably well with many flavors of bp, you may have a rare one that shows strong preferences. Your powder hasn't absorbed moisture has it? You're not using a powder charge beyond the capacity of the rifle to shoot well are you? If this might be the case, back off and see what the rifle "likes" to shoot.
3. Consider the use of an over powder wad. Some rifles dote on these, some don't much care.
4. Start the ball or bullet with care. If using a ball, center the patch carefully. It's easy to deform a muzzleloaded projectile enough to compromise resilts. If a ball starts too hard, change patch thickness or ball size. If a conical starts too hard, discuss swaging dies with a seasoned bp shooter, or consider changing bullets.
5. Seat the projectile firmly, but do not "whang" it or beat it with the loading rod! Again, it's easy to deform soft lead.
6. Seat the cap consistently. One thing to consider: I once ran into an inconsistent lot of RWS caps. Simply changing lots solved a mysterious accuracy issue.
regards,
Bob