Advanced Combat Optical Gunsight

Advanced Combat Optical Gunsights (abbreviated ACOG) are a series of telescopic sights manufactured by Trijicon. The ACOG was originally designed to be used on the M16 rifle and M4 carbine, but Trijicon have also developed ACOG accessories for other firearms. Models provide fixed power magnification levels from 1.5× to 6×. ACOG reticles are illuminated at night by an internal phosphor. Some versions have an additional daytime reticle illumination via a passive external fiber optic light pipe.

Design
The ACOG is available in a variety of configurations from the manufacturer with different reticles, illumination, and other features. Most ACOGs do not use batteries for reticle illumination,[2] being designed to use internal phosphor illumination provided by the radioactive decay of tritium. The tritium illumination has a usable life of 10–15 years.[3] Some versions of the ACOG have an additional daytime reticle illumination via a passive external fiber optic light pipe. Normally this allows the brightness of the reticle to match the field of view since it collects ambient light from around the sight, although this can lead to a mismatch in lighting - such as sunlight hitting the light pipe directly, or standing in a shadow - causing the reticle to be much brighter or darker than the target.[4] Reticles have other features such as a bullet drop compensator and other different reticle shapes such as chevrons.

Some ACOG models incorporate rudimentary ghost ring iron sights as a backup for targets that are within 50 m (55 yd). Most ACOG models, when mounted to a carry handle, have an open space through the mount to allow the use of the rifle's iron sights without removing the scope.[citation needed] Others include Docter or Trijicon reflex sights mounted on top. The ACOG ECOS line features both of these secondary sighting systems on the same scope.

Other features include rails, flip caps, and the ability to be waterproof up to 11 m (36 ft).