Ultimate Arms Gear Model 4/15 .223 5.56 7.62X39 300 AAC Blackout Rifle of 0.32mm Stainless Steel Free Float Freefloat Barrel Nut Ring Shims Washer 1 Shim Washer Designed for Model 4/15 .223 5.56 Freefloat Nut. Designed Where the Barrel Nut Ring Holds the Freefloat Into Place, Especially Vulnerable to Slight Product Tolerances, So these Shims can help to Make A Perfect Fit. Eliminates Gaps. These Washers Will Ensure a Contour fit Between Nut and Upper, Eliminating Misalignment and RIS Instability. FITS: .223, 7.62x39, Blackout AAC 300 // Will Not Fit .308 0.32mm Stainless Steel Shim Fit between the Nut and Upper to Allow the user to Adjust the Relationship Between the Gun and the Freefloat Features like Swivel Studs, Vent Openings to the Shooters Needs.