The TA31RCO is an Advanced Combat Optical Gunsight (ACOG) designed for the US Army's M4 weapon system (145 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 Designed to the exact specifications of the United States Marine Corps, the unique reticle pattern provides quick target acquisition at close combat ranges while providing enhanced target identification and hit probability out to 800 meters utilizing the Bullet Drop Compensator No tools are needed for windage and elevation adjustments because the TA31RCO features external adjusters, making it waterproof up to 11m without the caps Mounting bosses are integrated into the housing by the eyepiece to allow the easy addition of a Trijicon RMR