The MBUS (Magpul Back-Up Sight) is a robust and economical back-up iron sight solution.  The MBUS fits all Picatinny rail equipped weapons but is specifically tailored to the AR15/M16 system.  Folds to 5/8" tall and is spring loaded to firmly snap into up position when released.  The front flip up sight accepts standard AR-15 front sight posts.  The rear flip sight is adjustable for windage and utilizes both small and large apertures.  One click adjust POI (point of impact) approximately 3/4" at 100 meters.

Front and Rear Set of Flip-Up Sights.
Installs on standard rail systems. 
Comes with adjustment tool.