This is a Bravo Company Flash Hider known as the A2X chambered in 556NATO, model BCM-A2X.
The A2X(TM) is an extended version of the A2 flash hider found on the M16 and M4 series of rifles.
Features:
Threaded ½x 28;Machined from 4150 steel;Phosphate coated per MIL-STD-171;Predrilled hole at the base for permanent installation.