FG,
If I remember right that Deer Creek has open sights on it. 200 yards is a long way with regular sights, at least for me and my eyes. Your powder may be fine but the amount of sighting error could account for a large portion of your group size. I prefer a peep sight for those types of shots using my Lyman GPH. I've tried some 200 yard shots with open sights on one of my other sidelocks and was getting groups (lol) of around 15". Bear in mind that was with round ball so It loses quite a bit and it was windy. With the GPH and a 460 conical it will keep them in a 6" circle or better if I do my part. Good luck and let us know how it goes.