This high quality armorers tool is made from durable, NO-M.A.R polymer to protect the finish of your upper receiver. Simply insert this link between your upper and lower receiver and secure it with your rear pin and the provided hitch pin. Cleaning your barrel and receiver will be much easier without having to wrestle with your upper receiver while it rolls around on your work bench. NO-M.A.R polymer will resist all solvents and lubricants that a receiver might be subjected to, it will not soften or swell and can be easily cleaned for a long life.