Geissele Super Modular MK4 M-LOK 13.5in Rail - Black - Black - The Super Modular Rail (SMR) MK4 M-LOK is Geissele's ultra-lightweight M-LOK option. These rails are purpose-built and feature integrated M1913 picatinny rail sections at the fore of the rail in the 3, 6 and 9 o'clock positions. Maximum venting for heat dissipation also dramatically reduces the rail's overall weight, while maintaining its same strong and rigid platform. Utilizing Magpul's M-LOK technology at the 3, 6, and 9 o'clock positions, as well as the upper 45 quadrants. The SMR MK4 is both versatile and modular, allowing the user to tailor their platform to his or her unique needs. The SMR MK4 also features Geissele's proprietary 2.25in long barrel nut. This increases the surface contact where the rail interfaces with the receiver and creates a platform that can be trusted to be straight and true the entire length of the free float rail. This increases accuracy and zero repeatability during disassembly and reassembly when performing maintenance. Does Not Include Barrel Nut Wrench Type 3 Hardcoat Anodized 6061-T6 Aluminum