The AR-STONER AR-STONER M4 Collapsible Stock is perfect for upgrading an existing AR-15 or building a new carbine rifle. The M4 Collapsible Stock stock features a built-in sling swivel to mount your favorite rifle sling. The AR-STONER Stock comes ready to be mounted onto your AR-15 (Mil-Spec Diameter) carbine right from the package! FeaturesMil-Spec DiameterBuilt-In Metal Sling Swivel