This is a Primary Weapons Systems Compensator known as the CQB chambered in 308 Winchester, model 3CQB58C1.
The CQB Compensator is designed to redirect sound forward whereas others redirect to the side. Designed for close quarters combat" specifically law enforcement and military units shooting indoors and at close range" this compensator is gaining popularity for use on short barreled rifles to mitigate sound and blast" while improving overall weapon control.
Features:
5/8X24 Thread pitch
4140 Steel construction
Made for use with 300BLK and 7.62X39 caliber weapons