Sounds like to me you are pushing it a little too fast...My Knight 50 didn't like more than 90gr of 777 and a 250 Shockwave...You can get that group under and inch, just takes some work...

I found that my Knight liked the T/C sabots over the ones supplied with the Hornady's or the MMP-24s that I bought...

It also prefers either Goex FFF or Pyrodex R-S over 777...

I also have to not only clean between shot but I also have to lightly oil and then run a dry patch through the bore...

I can change any of the above three conditions and the groups go to 1 1/2 to 2 inches at 100 yards...

At 150 yards, it shoots 2 1/2inch groups, might not be worth it to some, but I like to shoot...