From the Trijicon website
TA31RCO-A4CP: 4x32 Trijicon ACOG® USMC Rifle Combat
Optic (RCO) with Red Dual Illuminated Reticle
The TA31RCO is an Advanced Combat Optical Gun sight (ACOG)
designed for the M16A4 Weapon system (20" barrel). It incorporates
dual illumination technology using a combination of fiber optics and
self- luminous tritium. This allows the aiming point to always be
illuminated without the use of batteries. The Tritium illuminates
the aiming point in total darkness, and the fiber optic self-adjusts
reticle brightness during daylight according to ambient light
conditions. This allows the operator to keep both eyes open while
engaging targets and maintaining maximum situational awareness,
Designed to the exact specifications of the United States Marine
Corps, the unique reticle pattern provides quick target acquisition
at close combat ranges while providing enhanced target
identification and hit probability out to 800 meters utilizing the
Bullet Drop Compensator.
TA31RCO-M4CP: 4x32 Trijicon ACOG® USMC Rifle Combat
Optic (RCO) with Red Dual Illuminated Reticle
The TA31RCO is an Advanced Combat Optical Gun sight (ACOG)
designed for the M16M4 weapon system (14.5 barrel). It provides the
shooter with quick target acquisition at close combat ranges while
providing enhanced target identification and hit probability out to
800 meters utilizing the Bullet Drop Compensator. The TA31RCO
incorporates dual-illumination technology using a combination of
fiber optics and self-luminous tritium. This allows the aiming point
to be always illuminated without the use of batteries. The Tritium
illuminates the aiming point in total darkness, and the fiber-optic
self-adjusts reticle brightness during daylight according to ambient
light conditions. This allows the operator to keep both eyes open
while engaging targets and maintaining maximum situational
awareness. Designed to the exact specifications of the United States
Marine Corp, the unique reticle pattern provides the shooter with
quick target acquisition at close combat ranges while providing
enhanced target identification and hit probability out to 800 meters
utilizing the Bullet Drop Compensator.