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Before they even become Marines, recruits must prove their aim is true with a rifle from 200, 300 and 500 yards downrange. The RCO began life as the Advanced Combat Optical Gunsight, or ACOG, and as it was adapted for the M16A4 and the M4, entered Marine Corps nomenclature as the RCO. With the AN/PVQ-31 RCO attached, Marines prove even more accurate, at even longer distances. The RCO is a fixed 4X optimal aiming scope that provides enhanced target identification and hit probability for the M4 Carbine and M16A4 service rifle out to 800 meters. Battery-free, dual illumination features enable Marines—in daylight, lowlight or no light—to keep both eyes open, while engaging targets and maintaining maximum situational awareness.

"The ACOG mounted on the M-16 service rifle has proven to be the biggest improvement in lethality for the infantry Marine since the introduction of the M1 Garand (the first semi-automatic rifle used in combat) in World War II."
Gen James N. Mattis, Commanding General of the 1st Marine Division during Operation Iraqi Freedom.