Absolute co-witness. Using the shortest mounting plate that came with the HS503GU, my reddot sits on top of my front sight post.
I took a 2 day PCC/Subgun class last summer, 1200 rds, in the bright sun and in the rain, shooting at distance, moving, on an obstacle course and in 2 shoot houses. I never had an issue with the sight picture being "too busy" as I have seen claimed. I kept my focus on the threat/target and just superimposed the reddot onto the target and fired. Plenty of brightness adjustment for bright sun, selectable reticle between just the 2 moa dot or the 2moa dot with 65moa circle, programmable auto shut down with shake awake, something like 25,000-50,000 on the battery depending on which reticle you choose and the brightness used. Uses common CR2032 batteries. I've got 3 now and there will be at least 1 more for my 9mm Colt AR.
