RE: Is it my shooting or something else?....
I would make a couple of changes...First, go to loose powder, I would start with Pyrodex R-S or Goex FFF if you can find it...Second, try another bullet, some guns just don't like PowerBelts...
Remember, you are handloading with a muzzleloader, every load needs to be consistant...That gun will shoot better than that, you just have to find the right combination...