Truckie - I wonder which model you have? I read a report on a web site somewhere that said the original Super Disc would not shoot 777 powder well. When he switched to the newer Disc Elite, everything was ok.
Why not try the bullet-powder combo that Knight recommends? I'm using Knight 175g Red Hots with 3 Pyrodex pellets and getting groups around 2" and mz velocity of 2425. This is with the older model rifle, but I've read that the newer model is more accurate. I'm sure if I went to loose powder and did a lot of experimenting I could find a better load, but I'm planning to hunt with this combo this season.
Good luck.