This VAMAR556 Ncstar AR15, M4 muzzle brake with crush washer screws onto standard AR15, M4 ½ " X 28 threaded barrels with .223, 5.56 bore. AR Muzzle brake has two chambers with 14 ports that direct the exiting propellant gasses upwards and to both sides to reduce muzzle rise and felt recoil. There are no open ports on the bottom of the muzzle brake this will reduce the dust signature when firing prone. Steel Construction with Black Finish. Length: 2.2" Weight: 2.7 oz.