I was determined to shoot Powerbelts and last year tried the 295 gr in my Black Knight. Couldn't keep bullets in a group less that 5" at ~100 yards. I gave up and shot the old faithful Knight 260 gr sabots, though not glamorous shot well and killed all that I hit them with.
This year, my dealer told me to try the heavier 348 gr Powerbelt, that it would shoot better from being a "longer" heavier bullet. Though I thought this was "horse hockey", I tried it. Man, was I impressed! I'm almost touching bullet holes at over 100 yards (100gr of the new "stick" Clean Shot rename). May not work for you, but as group consensus suggests, vary your bullets and powder until you find one that will shoot.

Rack