I would guess that the sabot is not compatible to the bullet and your bore. All muzzleloader makers are not tied to a "bore size for each caliber." Some rifle makers have a very tight bore while others have a much more expanded bore in comparison. Now we are not talking inches, we are talking much finer tolerances then that. So each .50 caliber bore depending in the maker and the rifle it self are not the same. For that reason you might have to try some different brands of sabot to get the Hornady SST's to work. I can not shoot a Hornady SST and a harvester sabot in my rifles but the same bullet and a MMP sabot will be a tight fit but work just fine.

Your powerbelt on the other hand is a slip fit conical. They are meant to go down the bore with little or no effort. In fact when hunting, check them from time to time and make sure they have not crawled off the charge. This is especially true of Knight rifles for some reason (bigger bore tolerance I will guess).