Look for .012 or .010 thick patching and try them. Always pick up fired patches on the ground in front of your muzzle, and check them to see if they are tearing, or have burn holes in them, etc. I had to take a file handle, wrap some emery cloth around the end of it, and smooth off the sharp ends of the rifling at the muzzle to get my balls to seat easier and without cutting the patch on those sharp edges. Then, I got some decent accuracy out of the gun.
If this gun is a keeper, its worth considering have the muzzle coned, to easy in loading the patch and round ball. That keeps the ball ROUND in the barrel, and not with a flattened shape from that sharp blow to the short starter. You are not going to notice the change in accuracy at 25 yards, or even at 50 yards. But at 100 yards, how you treat that ball when its loaded begins to show. I was very frustrated with my groups at the longer range until I spent an hour at the range at Friendship watching how the shooters loaded their PRB for the long range shooting. The jag heads were coned to perfectly fit the arc of the round ball they were loading, and they did not " whack " anything. The ball was centered, and the shorter starter was used to push the ball into the muzzle. Half the shooters were using pre-cut patches, and the other were still cutting their patches at the muzzle with a razor. The ball was carefully and slowly run down to a mark on the range rod to seat it on the powder. Their loading technique was subtly different from my own, but when I went home and tried their ways, it made a lot of difference in the groups at longer ranges.
