NcStar AR15 Flip Up Rear Sight (MARFLR) Precision made Flip-up A2 rear sight. The MARFLR will fit virtually all AR-15, M4, and M16 Flattop Picatinny rail upper receivers A2 rear sight Assembly with large and small rear Aperture. A2 rear sight is fully adjustable for windage with knurled click adjustable controls Large steel thumb nut and precise rail clamp for positive & secure attachment to your upper receiver