Flagaway&reg; .223 / 5.56 Free Float Rail Barrel Nut Washer Will fit .223, 7.62x39, Blackout AAC 300 Won't fit .308 This 0.32mm stainless steel shim fit between the barrel nut and upper receiver to allow the user to adjust the relationship between the gun and the forearm features like swivel studs, vent openings, and picatiny rails, to the shooters needs 10 pieces total