Powerbelts are made to cater to CVA/Winchester bores. It sounds like too much gas is escaping around the bullet on takeoff... stabilizing it out to 50 yards... but not 100. You also need to try 110, 105, 95 or 90 grains. If using pellets, buy the 30 grainers to adjust your load better.

You can also try filling the bottom hollow cavity of the plastic base with Bore Butter. That may seal the gas better. Odds are though, you'll get better 100 yard groups shooting T/C-packaged sabot/bullets like the Shockwaves, Mag Express Sabots or the Cheapshots.