The Trijicon ACOG TA31 Rifle Scope features Dual Illumination (Fiber Optic system provides daylight illumination and tritium illuminates reticle at night). The ranging reticle is calibrated for 5.56mm rifles out to 800 meters. ACOG(Advanced Combat Optical Gunsight) rifle scopes are internally-adjustable, compact telescopic sights with tritium illuminated reticle patterns for use in low light or at night. Bindon Aiming Concept (BAC)* models feature bright daytime reticles using fiber optics which collect ambient light. *This 4x32 model uses the Red Donut reticle and Mounts on the M16/AR15 carry handle. It can also be mounted on a flattop Picatinny Rail with the optional TA51 flat top adapter. ACOG rifle scopes combine traditional, precise distance marksmanship with close-in aiming speed. Although the ACOG rifle scopes have many features which are very advantageous for military use, they were developed by Trijicon without government funding. Designed for use with the M16 family, every feature of its mechanical and optical design was chosen for a single purpose: to provide increased hit potential in all lighting conditions -- without failure-prone batteries.