The TA31RCO is an Advanced Combat Optical Gunsight (ACOG) designed for the US Army's M4 weapon system (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;m which allows the operator to keep both eyes open while engaging targets and maintaining maximum situational awareness.