The TA31USN-M855 is an Advanced Combat Optical Gunsight (ACOG) designed for either the M16A4 weapon system (20” barrel) or the M16M4 (14.5” barrel). It incorporates dual illumination technology using a combination of fiber optics and self-luminous tritium. This allows the aiming point to always be illuminated without the use of batteries. The tritium illuminates the aiming point in total darkness, and the fiber optic self-adjusts reticle brightness during daylight according to ambient light conditions. This allows the operator to keep both eyes open while engaging targets and maintaining maximum situational awareness. The unique reticle pattern provides quick target acquisition at close combat ranges while providing enhanced target identification and hit probability out to 1,000 meters utilizing the Bullet Drop Compensator. No tools are needed for windage and elevation adjustments because the TA31USN-M855 features external adjusters, making it waterproof up to 11m without the caps.The TA31RCO-M855 includes:1 LaRue Tactical Mount (LT799)2 Assemblies, 1/10 Miliradian External Adjusters2 External Adjuster Caps1 Lanyard Assembly for Adjuster Caps (TA71E)1 Scopecoat (TA96)1 LENSPEN® (TA56)1 RCO Manual1 Warranty Card