This is a Knights Armament Company Bolt Carrier known as the SR-15/16 Sand Cutter, model 30091-1.
The Knight's Armament SR-15/SR-16 Chrome Sand Cutter Bolt Carrier comes with grooves machined into the carrier" to allow for free movement and operation in extremely adverse conditions.
Features:
Enhanced Reliability
Hard Chrome
Machined Grooves On Carrier Body