Flagaway&reg; .223 / 5.56 Free Float Rail Barrel Nut Washer 20 Pieces Will fit .223, 7.62x39, Blackout AAC 300 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 Won't fit .308 20 pieces total