Advanced Combat Optical Gunsights (abbreviated ACOG) are a series of telescopic sights manufactured by Trijicon. The ACOG is designed to be used on the M16 rifle and M4 carbine, but Trijicon has developed ACOG accessories for certain other firearms. Models provide fixed power magnification levels from 1.5x to 16x.[1]

The ACOG's reticle is illuminated by an internal phosphor illuminated by the radioactive decay of tritium. The tritium illumination has a usable life of 10-15 years.Some versions have a additional daytime reticle Illumanation via an external fiber optic light pipe.