Magpul Industries MBUS Back-Up Rear Sight Gen 2, Fits Picatinny Rails, Flip Up, Black MAG248-BLK The MBUS (Magpul Back-Up Sight) is a low-cost" color injection molded" folding back-up sight. The dual aperture MBUS Rear Sight is adjustable for windage and fits most 1913 Picatinny rail equipped weapons" but is specifically tailored to the AR15/M16 platform.