BT33 TUBE RAIL KIT The TUBE RAIL KIT, part number BT33 consists of a Steel Bar Nut that is designed to fit inside free float tubes and a low profile 1913 Picatinny rail 2.050" long with a 1.00 radius back and 1.602 between the two holes and two fasteners. This BT33 TUBE RAIL KIT is installed with the Steel Bar Nut inside the free float tube and the 1913 Picatinny rail on the outside with the matching holes aligned, fastened together, they allow the attaching of the BT10 or BT10-LW17 Atlas Bipod or other accessories to free float tubes.