This is a CMMG Buffer, model 90CA9B4.
The CMMG MK9 AR-15 9mm Carbine Buffer is a replacement buffer for AR-15's/M4's that are chambered in 9mm luger using a carbine receiver extension.
Features:
Fits AR-15 rifles chambered in 9mm Luger/CMMG MK9 or similar pattern variant using a carbine receiver extension
