I'll ask the question first then explain. Will moving a non adjustable rear sight forward or backward (on a level rib, its only ramped at the last couple of inches) change the point of impact?
My set up is an Rem 870 SM, sights are the Gobble Dot I think. POI at 40 is at least 8 inches high and to remedy the problem I either need to lower the rear sight or raise the front sight. I've considered shimming the front sight higher but question durability if I do that. The rear sight is already pretty low compared to most. Any suggestions will be appreciated!
What about taking a finer bead? In other words not letting so much of the front sight show in the groove of the rear sight. My 870 20 gauge will shoot about six inches low unless I show the full bead when sighting along the rib.
I'm not sure about your 870, but my Mossberg 930 was shooting high out of the box and it couldn't be resolved by adjusting the factory rear fiber optic. However, do to the fact that the rib does slop down as it approaches the receiver I was able to slide the rear sight back to drop the POI. Unfortunately, I didn't find this out until I had already ordered a set of Williams Universal Slugger sights so I went ahead and installed the Williams. To the eye, your 870 rib may not look like it slopes down in the rear but it wouldn't hurt to slide your rear sight back and give it a try. Good luck!
Buy all steel, adjustable magnum gobble dot sights..problem solved.