This MK4 jig is fabricated to fit a specific gas block, this model is for BCM and Vltor specific gas blocks. The MK4 fixed jig is fabricated from carbon steel and mounts to the gas block set screw holes using longer provided set screws. The drilling guide hole is located on the OD of the barrel to provide 50% pin diameter in the barrel. Stock drilling hole size is #31 (0.120"). The jig is drilled to mount the barrel at a 1" center. The Jig will include needed set screws and nuts and one #31 HSS drill. Mfg: Black Rifle Disease Engineerin