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.Features : Spring loaded flip up sight Spring-loaded flip up sight easily activated from either side or by pressing the top Protective wings shield the dual, same plane flip apertures Detent knob easily and positively adjusts windage Polymer construction provides light weight and resists abuse Detent and spring pressure keeps sight in place but allows for unobstructed folding under impact