Consistancy and small increments are the key.
Let me recommend that you take whatever bullet/sabot combo you have,
Start with the powder you have and start at say 80grns.
Clean the barrel, dry it and load your first set.
Make sure the sabot (w/o any lube) is seated on the powder
DO NOT TAMP the rod steady firm pressure. Try and get some kind of group with this, then experiment with the charge in 5 grn steps
and see what you can do. I had really bad results with power belts
and had to switch to a sabot round. You could check with the manufacturer for a recommended round, you also might want to
bring it to a gunsmith to check the rifling and crown
Good luck and keep us posted