I've never been able to get good consistant accuracy with powerbelts. I've tried them in a couple rifles that did OK for a group or two, then "fliers" started showing up. Not sure of the charges I was using, but I usually start off with around 80 grains of loose powder for any load I test. I like the theory behind powerbelts, I like conicals and they are basicaly a conical with a gas seal attached.
For example - with my T/C NewEnglander I tried powerbelts. 1st group @ 50 yards was about 1" and I thought I'd found a winner. Next group was about 6"! Same rifle, same shooter, same load, why the huge difference in results?????