This 9mm Caliber Blow-back System Bolt Carrier is necessary for proper functioning with 9mm Ammunition in our Windham Weaponry AR platform Multi-Caliber system. Unlike the two separate components of the standard .223 Bolt and Bolt Carrier, this 9mm unit is one solid assembly. It includes a firing pin and the staked on guide piece on top. It should be used with our heavy 9mm Buffer (see P/N 1005-914-4578-9MM ).