The Atlas Flush Cup 4" Picatinny Rail will accommodate hole patterns with a center to center spacing of 2.2 - 3.5" with a flat base to accommodate flat surfaces. A standard sling stud can be used as one fastener or two button head machine screws can be used (not included) to attach this rail to the forearm of standard rifle stocks. This rail can also features a built in flush cup bushing for QD sling swivels.