I recently purchased a Fully rifled Ithaca replacement barrel for my Remington 870, I am unable to get a point of aim, point of impact at 40 yds using 3 inch sabot slugs. 40 yds is ideal for the terrain I hunt to include the abundance of wild hogs which a slug is ideal. The rear sight can not be adjusted low enough to allow me to achieve point of aim point of impact.At this distance I must hold the sight 8 inches above the bullseye. Any suggestions would be appreciated.