This is a Aero Precision MB known as the Epsilon chambered in 223 Remington, model APVG100025A.
The EPSILON 556SL is the ideal muzzle brake for 14.5" rifle builds. Featuring an outside diameter of .745"" the EPSILON 556SL can be pinned and welded to your barrel while still allowing full maintenance and removal of your gas block without having to remove the muzzle device.
Features:
Reduces felt recoil significantly
Aggressive chamber design improves gas flow
Deep channel cuts reduce weight