I also have a Knight Disc...22 inch barrel, with a Leupold 3x9x40...I have had this gun since 1999...I have probably shot this rifle 500 times...My first pet load was 2-50gr Pyrodex pellets and the 300gr Barnes copper bullet...The problem with shooting a hollow point bullet in an inline is when you get out to 150-200 yards...
They are not aerodynamic, the front of them is flat, or blunt, this is excellent for mushrooming, but not good for longer range shooting...So, the PowerBelts, Hornady XTPs, 250 and 300 gr Barnes Hollow points, and many of the other hollow point, pistol type bullets aren't going to group well at these ranges.

I have shot flintlocks for about 30 years, so I am used to measuring powder charges...Three years ago I started really fine tuning my Knight...I tried several different bullets, including all of the above, I also tried 777 loose and pellets and Goex FF and FFF and Pyrodex R-S....I also tried the Hornady SST (T/C Shockwave bullets) and ordered MMP-24 sabots....I can now group 250 SST's into one ragged hole at 100 yards, and into a 2 inch group at 150...I'm 2 1/2 inches high at 100 and 2 1/2 to 3 inches low at 150...I use 90 grains of Pyrodex R-S...Out of a 22 inch barrel I know my velocity is fairly low and 150 is pretty much my max range....

Now...while I was doing this experimenting I also tried PowerBelts and the 300 gr Barnes, sighted in 2 1/2 high at 100...I was 6-8 inches low at 150 and the groups opened up to about 6 inches.....

So...what I'm saying...Pick a bullet for the max range that you will be shooting....
Of you will be shooting longer ranges, taylor your powder charge to the gun and bullet....I prefer loose, because you are handloading each load and you can greatly improve the accuracy by varying the powder charge...

If you are going to shoot past 100-125 yards...Buy a range finder...Last fall,I was hunting over one of our soybean fields and shot an 8 pointer at 143 yards...I held just below the top of the back, hit him in the shoulder blade, and he dropped....All that experimenting paid off right then.

There is nothing wrong with using pellets and the Hornady XTPs, or the PowerBelts...Especially if you are hunting in the woods, and shots are within 100-125 yards or so....But...Once you move out to 150-200...It takes a little more effort to find the proper load.