about those powerbelts:
They designed powerbelts to fit cva's perfectly, they calibrated the bullet diameter to fit the inside diameter of the cva barrells perfectly, other than that most other guns don't shoot them to well.
About bullett penetration and expansion:
I believe one of the problems may because your bullet weights are 250-300 grns they may not be expanding cause they blowing through so hard. I think that 200 grn bullett of mine may have expanded so well and so rapidly due to the lower grn weight of the bullett. Just something to ponder.