You should not be using 4F powder in the barrel. This is priming powder ONLY! Its lucky the barrel has not blown up. Of course the patch is going to burn!
Measure the bore with calipers to determine the exact diameter from land to land( bore diameter.) Then measure the groove diameter. Or call Mark and ask. He makes excellent barrels. Use a ball that has a diameter at least .010" smaller than the bore diameter. The groove diameter and groove depth indicates how thick a patch you might need in that barrel. Lubricated patches do compress about 33%, and the grooves allow some of the cloth that would otherwise be between the ball and the lands to " squish " out into the grooves during that compression, helping to seal the bore from gas.
A better way to get consistent accuracy is to use a fiber wad as an OP wad between the powder and PRB. A filler of corn meal, cream of wheat, hornet's nest, etc. can also be used to seal the gases to protect the patch and ball from gas cutting and burning.