Odds are if you’ve found this it’s becasue you’re building a clone rifle of some sort.  This is accurate/correct for use on the MK18/CQBR and M4A1 as well as the legendary MK12 rifles.  Buy the best!

MFG Description

Slim, low-profile replacement for factory front sight gas block provides maximum clearance for optics and free-float handguards. Streamlined profile reduces forward weight and improves your rifle’s appearance. Set screws, gas tube mounting pin, and installation instructions included. Mk 12 is an exact copy of the gas manifold on the Mk12 Mod 1 and Mk 18 Mod 1 Special Purpose Rifles used by the U.S. Army and Navy. Made from 416 stainless steel with a tough, wear-resistant black oxide finish, and fits barrels with .750″ O.D.