I have not had a problem shooting the 250 sst or shockwave in either of my muzzleloaders,using 100 grs of pyro select.But have found that sometimes the rifle and sst's preferr the short sabots.If lowering the powder charge didn't work.
I would take 3 of the sabots that come with the sst,s and cut off the pedals making the sabot about .755 in long in total length,and see how they group. If they work you can buy the shorter sabots a lot cheaper.If that didn't work I would get some JB's bore cleaner and give it a good cleaning making sure all the plastic is removed from the barrel.
If that don't work you will have to try someting else sometimes a rifle just won't shoot certain bullets no matter what you try.