The BT15 - 3.35 inches 1913 picatinny rail is designed to fit hole patterns with center-to-center spacing ranging from 1.76 inches - 2.78 inches. It features a relieved back to accommodate curved surfaces and includes a flush cup for the use of flush cup sling swivels and sling studs (not included). You can use a standard sling stud and sling stud spacer as a single fastener or opt for two button head machine screws. Please note that no hardware is included. For attaching this rail to the forearm of standard rifle stocks, refer to BT1517. It is machined to picatinny rail standards from 6061-T6 aluminum and finished with a mil-spec type III class 2 coating. Available exclusively in black. The Atlas 1913 rails are engineered to facilitate the attachment of Atlas Bipods to picatinny rails, featuring a 3.35 inches rail option specifically designed for curved surfaces To install the ACCU-SHOT picatinny rail adapter, a sling stud and spacer or machine screws are required The 3.35 inches rail is compatible with hole patterns ranging from 1.76 inches - 2.78 inches center-to-center Additionally, a separate 4 inches rail option is available for flat surfaces with spacing from 2.2 inches - 3.5 inches