The MBUS (Magpul Back-Up Sight) is a low-cost, polymer, 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 easily activated from either side or by pressing the top, detent and spring pressure keeps sight erect but allows for unobstructed folding under impact; clamps to MIL-STD-1913 Picatinny/STANAG 4694 receiver rail and provides the same height-over-bore as standard A2 irons and protective wings shield the dual, same plane flip apertures. Sight can still be folded with either large or small aperture in position. Detent knob easily and positively adjusts windage. Made in the USA.