I used some 777 3f in my Knight Disc...Found out it didn't like more than 90grs, also had the problem with the crud ring...So much so that I had to lap the barrel to remove, I quit using it...I get better results with Pyrodex R-S (found it on sale at WalMart for $5)...But after I finish what I have I'll probably go back to Goex 3F...I can buy that at the distributor for $11.55 a pound and since I also have 2 flintlocks I need that anyway...
Bass Pro has Goex in Charlotte, but you have to ask for it because it has to be stored in a fireproof safe...only problem is they want about $20 a pound for....
If memory serves me right, you bought the Knight Disc...I believe its rated for 120 grains 777 loose....Look at the Knight website and get their number, I've called them a few times and was always able to get a real person on the line and they are very helpful....
My current loading is 90 grains of Pyrodex R-S or 90 grains Goex FFF and a 250 T-C Shockwave (or Hornady SST)....I have also had good groups with 100 grains and a 300 Barnes copper, just quit using because of their high cost and the SSTs are more aerodynamic, so I sight in 2 1/2 high at 100 and I'm 2 1/2 low at 150....