I really think you are pushing the powerbelts too hard. Drop them down to 80 grains of loose Pyrodex RSpowder and try them. All too often we see this magnum snake head raise up. Just because it is a magnum capabile rifle, you do not have to shoot magnum charges.

If sabots are allowed, try some XTP's or Shockwaves. All of this is a matter of trial and error.