The MARDRS will fit virtually all AR-15, M4, and M16 flattop Picatinny rail upper receivers. A2 rear sight assembly with large and small rear aperture. The A2 rear sight is fully adjustable for elevation & windage with knurled click adjustable controls. Large steel thumb nut and steel rail clamp for positive & secure attachment to your upper receiver. Compact enough, where you will still be able to mount a red-dot/ reflex/ holographic optic on your AR-15 upper receiver. Precision made A2 rear sight. The mount and rear sight is made from high quality aluminum and steel.