Altra Arrows 166 Insert O-Ring Silver. R.C.O.S. are essential in the insert/outsert system as they aid in locking the outsert in place reducing vibration and improving concentricity between the outsert to the insert for a more precise arrow set up and down range accuracy. Specifications for Altra Arrows 166 Insert O-Ring: Manufacturer: Altra Arrows Color: Silver Features of Altra Arrows 166 Insert O-Ring: Comes in 12 pk QTY Package Contents: Altra Arrows 166 Insert O-Ring Altra Arrows AA-AC-00015: 166 Insert O-Ring Silver