Welcome aboard, haven't been posting here too long myself, but now I can't get enough of it...great people, great info, what more could you ask for?
Have the same gun and had pretty much the exact same problem that you are having. Before I bought It I talked to a bunch of folks that already owned one (though theirs were blued, for whatever its worth)and they couldn't say enough about the accuracy with powerbelts. I believe one of them even offered me all of their other misc. bullets cause nothing else even came close. Sure enough I stock up w/777 and powerbelts and head down to the range, needless to say, one hell of a sore shoulder later, I'm ready melt the barrel into an anchor and go back to the iron sighted sidelock.
Out of sheer anger and stubbornness I try the T/C 240gr. (xtp?) and the 300gr Hornady XTP. Both shoot 1 1/2"-2 1/2" groups with me pulling the trigger!! 110gr of loose 777 and I dropped three deer in their tracks!! (even if two were pretty small :oops: ... that's another story)
Long story short, :roll: I'm not sure if it was barrel break-in since I haven't tried the powerbelts since, but both petal style sabots worked great even if they are harder to load than the P.B. bullets. I do dry brush between shots at the bench, and pay special attn. to the crud ring. Hope this helped(even if it was a bit long winded ), I can't sleep, It's late and writing about the whole sordid mess kinda makes me feel better.