BT15 ATLAS 3.35" FC 1913 RAIL 3.35" 1913 RAIL HOLE SPACING FROM 1.76" TO 2.78" A STANDARD SLING STUD AND SLING STUD SPACER CAN BE USD AS ONE FASTENER OR 2 BUTTON HEAD MACHINE SCREWS. NO HARDWARE INCULDED.SEE BT1517 FOR ATTACHMENT HARDWARE. WEIGHT .92 OZ.