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" 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.Bundled with a standard Super Gas Block - The bore of the SGB is machined to closely fit around .750" diameter mounting bosses found on most M4 Carbines and AR-15 rifles..750" Low Profile Stainless Steel Geissele Super Gas Block IncludedGeissele rails are designed to work with Mil-Spec M4 upper receivers. The compatibility with billet receiver sets is unknown