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.

• M1913 picatinny rails in the 3, 6, 9, & 12 o'clock positions
• M-LOK slots in the 3, 6, and 9 o'clock positions & in the upper 45 quadrants
• Lightweight design
• Maximum ventilation on the barrel