The ACOG RCO QD Mount is designed for Trijicon TA31RCO (Rifle Combat Optic) ACOG scope, which is essentially a TA31 with a special reticle designed for the USMC. There are different variants of the TA31RCO; one of them is for the M4A1 and another for the M16A4. One of the issues with the M16A4 in service with the USMC is the non-collapsible, full length A2 stock. The long length of this stock can cause issues when wearing body armour as the stock is too long to allow proper eye relief with magnified optics.
