My youngun had problems with the PowerBelts out of his also. They were shooting all over the place until he dropped back to 2 50gr Pyrodex pellets. He was shooting the 295gr ones. Saboted bullets, so far, haven't done all that good for him, but that was while he was messing with 150gr of powder.
I got to shoot a 700ML yesterday. Saboted 240gr XPTs weren't shooting very well from it. I was shooting 100grs of Pyrodex powder, not the pellets. Do you shoot the pellets from yours? This rifle uses caps, not 209 primers and I didn't know if the caps would ignite the pellets consistently.
He took me!
Wish you were here.
Proud Parent of a Marine