This is a Yankee Hill Machine Co Muzzle Brake known as the Slant Brake/Compensator chambered in 556NATO, model YHM-26-MB-A.
This 5.56mm Muzzle Brake and Compensator hybrid is designed to reduce recoil as well as muzzle rise for a quicker follow up shot.
Features:
1/2X28 thread pitch
QPQ melonite finish
Includes crush washer