Installed on a fn 509cc - Holosun EPS-Carry 1x0.58x0.77in Reflex Red Dot Sight w/Multiple Reticle System

Good sight. would buy again. Note it did take 5 minutes with my dremel tool to open the front Holosun mount plate holes slightly for the FN lugs to fit the snug. I would say Holosun got it right and FN was a little off on even lug size by my measurements. None of the 3 sets of supplied screws worked. Either length or thread pitch was incorrect. I used a set that came with my FN's selection of mounting plates and screws. In the end it came out great. Co-witness with the factory FN sights are perfect.