BUFFER SPRING: CarbineBUFFER: CarbineMAG RELEASE: Mk9BOLT CATCH: Mk9FEED RAMP: ASC/CPD Mag OptimizedCMMG has made a small modification to 9mm barrels in order for them to be compatible with Mk9 lower receivers. 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.