CMMG Receiver Extension A1 Black. Made from black hard coat anodized aluminum the CMMG A1 receiver extension (buffer tube) is designed to accept fixed rifle length stocks and is compatible with Mk4 (AR-15) and Mk3 (LR-308) lower receivers. Specifications for CMMG Receiver Extension A1: Manufacturer: CMMG Color: Black Gun Make: AR Platform Gun Model: AR-15 Package Contents: CMMG Receiver Extension A1 What firearm models does the manufacturer say CMMG Receiver Extension A1 is compatible with? The manufacturer says that CMMG Receiver Extension A1 is compatible with AR-15CMMG 55CA9B7: Receiver Extension A1 Black