The Super Modular Rail (SMR) MK4 M-LOK is a rugged, versatile, and modular rail system from Geissele. It features integrated M1913 Picatinny rail sections at the fore of the rail in the 3, 6 and 9 o'clock positions, as well as maximum venting for heat dissipation. The SMR MK4 also features Geissele's proprietary 2.25 inches long barrel nut, which increases the surface contact where the rail interfaces with the receiver. This creates a platform that is trusted to be straight and true the entire length of the free float rail, increasing accuracy and zero repeatability.