I completely agree with Old Colonel, first ask H&H on the original configuration then start pattern testing different loads using different shot cups as opposed to the modern plastic wads to tighten the pattern.

My older Pedersoli muzzleloader had cylinder bores and while shooting another black powder shooter said that with a shot cup you could tighten that pattern measurably. My Pedersoli already was throwing tighter patterns when I was using a heavier shot charge and a lower powder charge. I never did try the shot cup but he said it worked on his muzzleloader.

http://www.possibleshop.com/s-s-wads-shotgun.html