ALTRA INSERT AND OUTSERT .166 350 SPINE The insert secures into the arrow shaft at a depth of 0.875", while the outsert then threads directly onto the insert and encapsulates the leading end of the arrow at 0.43" creating unmatched durability. Essential rubber O-rings come pre-installed on the insert within the A.R.C.O.S., as they aid in locking the outsert in place which reduces vibration and improves concentricity between the outsert to the insert for a more precise arrow set up and down range accuracy.