The MK12 Gas Block is only compatible with the RIS II (M4A1 & MK18) family of rail systems manufactured by Daniel Defense. It will not fit within any other Daniel Defense rail system. This is due to various clearance issues and only applies to rail system lengths that would completely shroud the gas block. If you are mounting the gas block to the barrel without being shrouded by the rail system, then this does not apply. In that case, it will fit as long as barrel diameter at the gas port is .750". Due to extremely tight manufacturing tolerences, some fitting may be required to for installation.