This is a CMMG Buffer, model 55CA977.
CMMG manufactures high quality parts and accessories for the AR-15 platform. This CMMG AR-15 Buffer Assembly is a replacement buffer for AR-15's/M4's that are using a carbine receiver extension.
Features:
Fits AR-15 rifles using a carbine receiver extension
Mil-Spec
Buffer is used in CMMG6 Position M4 Stock Kits
