Altra Arrows 166 Spine Insert and Outsert Heavy. R.C.O.S.) for 166 Shafts is designed to deliver unmatched durability and performance. Crafted from 7075-T6 aluminum this system includes an insert that secures into the arrow shaft at a depth of 0.875 inches while the outsert threads directly onto the insert encapsulating the leading end of the arrow by 0.43 inches. Pre-installed rubber O-rings help lock the outsert in place reducing vibration and enhancing concentricity for a more precise arrow setup and improved downrange accuracy. Available in four spine sizes the A.R.C.O.S. system is perfect for those seeking optimal fit and performance. Specifications for Altra Arrows 166 Spine Insert and Outsert Heavy: Manufacturer: Altra Arrows Color: Grey Features of Altra Arrows 166 Spine Insert and Outsert Heavy: Fits Altra Arrows 166 Shafts Machined from 7075-T6 aluminum for unmatched durability Secure insert depth: 0.875 inches Outsert encapsulates arrow end: 0.43 inches Pre-installed rubber O-rings reduce vibration and improve concentricity Comes in 12 pk QTY Available in four sizes: 250 300 350 & 400 spine 50gn Total Weight Accepts micro FMS insert weights Enhances downrange accuracy and precision Package Contents: Altra Arrows 166 Spine Insert and Outsert Heavy