These mount adapters from Trijicon allow you to mount the MRO optic to your firearm. Three different models allow you to get the optimal optic height to fit your needs. Full Co-Witness mount is great for AR-15 style rifles and will perfectly co-witness with back-up iron sights. Lower third height gives you just enough height to allow your BUIS to co-witness in the lower portion of the optic so as not to interfere with your red dot.