On many AR-15s that I see, many of them have a set-up where they have the traditional fixed front sight as well as a red dot sight such as an aimpoint. Does co-witnessing through a holographic sight along with an iron sight make things difficult or annoying? Why not simply get a flip up sight as the front iron sight instead of a fixed sight post? Is there a reason for this? I've never shouldered a weapon that has co-witnessing sights so this could be a stupid question.