UCObaseball

You never said " how you cleaned" !!!

Are you using a nitro-type solvent for the final cleaning?
Are you using a bore brush to clean? If you do not rid the riflings of old, hardened bore butter, your bullet will not engage the rifling properly. I suggest using bore butter as a patch/conical lube ONLY... if not, throw it in the garbage.

Another suggestion
Loosen the stock to barrel bolt(s) around 270 degrees (3/4 complete turn) -- then push down on the muzzle side of the barrel while the recoil pad/buttstock is squarely on the floor. Holding down with pressure, tighten the bolt(s).

Suggestion part-4

Your Powerbelts may be defective. The ol' plastic may not be disengaging after take-off. Try a similar weight sabot/bullet. Powerbelt " bullet" diameter tolerances at the base need to be more " exact" than other sabot/bullets or conicals. I' m sure Powerbelt Inc/BPI checks them like clock-work but you never know! If it turns out to be .001 plus or minus... accuracy shifts directions & the cup/base will either be too loose & disengage prematurely or end-up going for a long journey with the bullet.