CMMG Parts Kit Mk9 9mm Radial Delayed Blowback Colt/Metalform Mags. This modification only applies to Mk9 platform lower receivers and does not positively or negatively affect functionality of the barrel with a MkGs Mk4 or Mk17 platform lower receiver. Barrels that are stamped 9mm Luger will be compatible with an upgraded Mk9 lower receiver. If your barrel is marked 9x19 NATO you will need to purchase a new 9mm barrel in order to ensure reliable function. Specifications for CMMG Mk9 9mm Parts Kit: Manufacturer: CMMG Color: Black Finish: No Caliber: 9mm Luger Gun Make: CMMG Gun Model: CMMG Mk9 Gas System Length: Carbine Length Flutes: No Package Contents: CMMG Mk9 9mm Parts Kit CMMG 91AFF52: Parts Kit Mk9 9mm Radial Delayed Blowback Colt/Metalform Mags