The BT15 – 3.35″ 1913 Picatinny Rail will accommodate hole patterns with a center-to-center spacing of 1.76 – 2.78″ and a relieved back to accommodate curved surfaces. It also has a flush cup allowing the use of Flush Cup sling swivels and sling studs (neither provided). A standard sling stud and sling stud spacer can be used as one fastener or two button head machine screws can be used. NO HARDWARE INCLUDED.