when using black powder you weigh it by volume not by weight, you dont or I dont know of anyone who uses a smokeless powder scale for their bp loads you ned to get a bp powder volume scale sold at most wal-marts or gunshops, b-4 you go any further I would look into it a little more as i am not the most up to date on this but you could be doing something that may be wrong. 
JIM
JIM
