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.

If you are hitting 8" below where you are aiming, then your sights are too low already!! Raise the rear sight up to bring the POI up to where you POA is. Good luck.

charlie brown is right. I think you are adjusting your sights in the wrong direction.

Yep, raise the rear sight if you want the bullet impact to move up.

For a rear sight (rifle) adjust it opposite the direction the projectile is impacting. Hitting low move the rear sight up. Hitting high move the rear sight down. Hitting left move the sight right. Hitting right move the sight left.

For a front sight (bow) you adjust the sight in the same direction that the projectile is impacting.

Good advice from the others.