223/6mm - Good for any bullet caliber up to and including 0.243 (6mm), ie 223, 5.56 NATO, 204, 17 cal, etc The HELLFIRE 2P Muzzle Brake is designed to provide an optimized brake for Semi-Automatic and/or hunting weapons systems with barrels of 20a ^2 or less. This brake will offer the greatest performance possible while maintaining shooter comfort. Part of the Patent Pending HELLFIRE Modular Brake System The AREA 419 HELLFIRE Self Timing Brake system is not just another knockoff copy of existing self timing brakes. It was engineered from the ground up to be extremely effective, easier to install, and much more versatile. Read more on all of its advantages below: Caliber Selection In order to simplify the choices, they have two caliber selections available for the HELLFIRE 2P brake. The difference in efficiency between using a caliber specific brake bore, and the next size up is so minimal if not non existent that we decided to simplify things in order to lower overall costs to you the consumer. As you can imagine, trying to stock the exact quantities required of six or so different calibers would be impossible. Notable Features - Extremely easy installation, brake WILL NOT turn as you torque the collar to lock. No more guessing where the brake ports will line up once you tighten it down! - Simple round knurled nut that requires no tools to time the brake. - Return to zero is excellent. This means removal of your brake and re-installation will not require a zero change. - Simply torque the universal adapter onto your barrel with a 5/8a ^2 wrench, this piece never has to come off. Next, LEFT HAND thread the brake/collar assembly onto the adapter until it is almost snug. Now rotate the brake until it is level, push the brake into the adapter and you will feel it lock onto the taper. Now tighten the collar fully by hand, and you are done.