Geissele Automatics MK4 Federal, Rail, OD Green, MLOK, AR15, 10 05-430ODG Geissele's Super Modular MK4 Rail is used by the Federal Bureau of Investigation due to its strength" modularity" and M-LOK compatibility. It features M1913 picatinny rails in the 6 o'clock and 12 o'clock positions" as well as shorter M1913 picatinny rails in the front 3 o'clock and 9 o'clock positions.