I can help. Try some Hornady or Knight high speed sabots with a 240 grain or heavier 44 or 45 pistol bullet made from hard lead. Drop down to 2 pellets and try different bullets until the rifle will hold 1 inch 3 shot groups at 50 yards(mine will). THEN try the 3 pellet load. Failing that, try the 30 grain pellets which can be added up to the 150 grain max. Sometimes 150 is JUST TOO MUCH for the machining on the gun. 4 (30) grainers is 120 and maybe the gun will stabilize with that charge. I have shot the Powerbelts, and they simply don't impress me with their sometime lack of accuracy. A friend has been trying to get his T/C to shoot them, and he finally listened and tried my loads. He is now one happy shooter, and saving money shooting my hardcast bullets instead of the Powerbelts. Sure they look kewl, but they might not be compatible with the rifle and pellet count. Not everything is really "NEW AND IMPROVED". Just marketing. Jim Shockey uses Noslers; try them too. Remember that BP barrels take awhile to break in; use Bore Butter for a bit of lube and the process will be sped up.
If you want your children to follow in your footsteps, be careful where you walk.
Beware the man that only owns one gun; he probably knows how to use it.
I just hope my ship comes in before my dock rots.