Trijicon ACOG, 4x32, Dual Illuminated Red Chevron, USMC Rifle Combat Optic (RCO) for A4 (20 in. barrel), With TA51 Mount TA31RCO-A4CP The Trijicon ACOG (Advanced Combat Optical Gunsight) is a fixed power, compact riflescope with an illuminated reticle pattern for use in bright to low/no light. The ACOG is designed to be extremely durable and reliable. Combining traditional, precise distance marksmanship with CQB speed, many variants include a bullet drop compensated (BDC) reticle. Every feature of its design was chosen for a single purpose: to provide increased hit potential in all lighting conditions.