Cvillanueva, Co-witness is the ability to continue to use your IRON SIGHTS through your NON-MAGNIFICATION OPTICS. In other words, you have a Red Dot, your in battle, the battery dies, now you have an optic that has no Red Dot, you flip up the front and rear IRON SIGHTS and sight right through the optics, putting you back in the fight. It is also refered as BUIS or MBUS for Magpul Back Up Sights. Always have a backup when using electronic assisted optics. Hope that helps. Military and Law Enforcement use this setup.
Can't use it if your weapon has magnification, exp. 4X scope.. You will see ARs with scopes and no iron sights sometimes unless your scope is quick release and you remove it, or you have a high rise scope mounts with holes so you can sight through it and use your iron sights below the scope mounts.
Note: There is primary two kinds of co-witnessing, FULL and 1/3 co-witness. Full is the your optics is low mounted on the rail, when your BUS is flipped up, the center is in the middle of the optic. 1/3 is your optic is raised up 1/2" and when you flip up your BUS, you sight through the lower part of your optic (6 oclock) area. Both work, you have an option as to where you want your optics.. Good luck.
