Accu-Shot 3.35" 1913 Rail (BT15) Rail will accommodate hole patterns with a center to center spacing of 1.76 - 2.78" relieved base to accommodate curved 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 The BT15 can also be attached to a horizontal surface on the Butt section to attach our BR12 or BT13 Precision Rail Monopods. The BT15 also has a flush cup for that style of sling swivels.