Like has already been said.......................try more bullet & less powder.
I shoot the 348gr aerotips with 90gr of loose 2fg 777 and get sub2" groups at 100yds out of my Knight. A 100gr 777 pellet load should give you close to the same results.

Also, here's a tip for shooting powerbelts.....................Loosen the cups from the bullets some before shooting them. Typically I twist them off & on a few times and put just a little dab of lube on the bullet's post beforeI shoot them. I've found that if you push the powerbelts a little too hard, the cups won't release from the bullets and that throws the accuracy to the "four winds". If you ever see a BIG hole in your paper target when shooting powerbelts - you'll know the cup didn't release. But even if they area little sticky and don't release as quickly as they should - accuracy will suffer some.