Magpul Mbus Rear Back-Up Sight MAG248 Black - 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.
Made in U.S.A.
FEATURES
Impact resistant polymer construction provides light weight and resists operational abuse
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, etc.
Clamps to MIL-STD-1913 Picatinny/STANAG 4694 receiver rail and provides the same height-over-bore as standard A2 irons
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
Specs
SPECIFICATIONS 
MBUS – Rear
Weight, w/ hardware: 1.3 oz.
Height above rail, stowed: ~0.51 in.
Center of aperture to top of rail: 1.42 in.
Length, stowed: 2.6 in.
Length, deployed: 1.5 in.
 
WINDAGE ADJUSTMENT
~0.7 MOA (0.754"/100m) per click with a 14.5" sight radius
~0.5 MOA (0.547"/100m) per click with a 20" sight radius