I've never tried 300g bullets in my Huntsman but I was at the range yesterday checking my sights for the up coming early muzzleloader season. My Huntsman is equipped with 2x7 32mm Pine Ridge scope from Cabelas.
I was using a 240g bullet/sabot from Knight rifles(they were on sale) over two 50g Prodex pellets. I was 80 yards from my target. My first two shots touched each other about 2 inches dead above the point of aim. The third shot opened the group to about 2 inches.
I adjusted my scope and put two more shots into the 1 inch bulls eye. That was good enough. I cleaned the rifle and but her back in the case. I think I'm going to stick with the 240's this season and see how the do.
BTW, I was shooting with a guy that was using a much much more expensive muzzleloader and scope and his groups were no smaller than mine.
I really like that little gun. I can't wait to see what it can do in the woods.