When I got my 50cal. Knight, first thing I tried was Power Belt 245gr. AeroTips....from a clean barrel, or even after 2 shots, you never knew where it was gonna go!....after 3 or 4 shots they would group pretty good.
I contacted PB, and they told me that Knight produced barrels that were slightly "over bored", and that the "lighter" PBs did not perform well thru them, because the "belt" would not properly seal in the larger bore.... They suggested going to the heavier bullets (295gr. or 348gr.) which would provide more resistance, and make the belt seal better.
Instead, I chose to try the T/C 250gr. bonded Shockwaves (blue tip)....w/ 100grs. of 777, and have had real good results. These are harder loading than the PBs, but at least I know where my first shot is going!