Quiet Control Brake for 6.8. 5/8-24 tpi mounting The QC Brake was designed to be comfortable to shoot with earplugs only, not requiring muffs as so many brakes do. Concussion is reduced behind the muzzle to make shooting comfortable for the shooter. Another design feature is neutral compensation. Whether firing one round, or a full auto mag dump, the QCB tracks on target, no muzzle rise or nosedive. Great for competition rifles, The QC Brake is only 2" long and just under 1" in diameter, so it is legal for all USPSA divisions.