This is a CMMG Receiver Extension known as the Enhanced Buffer Tube, model 55CA65A.
When you are thinking about building a new AR-15 pistol" CMMG's Pistol Receiver Extension Kit is what you need to keep that lower receiver pistol compliant. CMMG has created the perfect kit containing an AR-15 pistol receiver extension" ambidextrous sling end plate" carbine buffer and spring. This kit is designed to work correctly with both .300 AAC blackout and 5.56mm NATO AR pistol builds.
Features:
Fits .300 AAC Blackout or 5.56mm NATO pistol builds
