These fit the plan B type mounts. This is an updated version that has larger wrench flats on the rear. Always use an alignment rod to ensure that the entire system is set up correctly. Make sure all surfaces are cleaned and the muzzle device has been torqued and Rockset to the barrel. Torque the muzzle device to approximately 15-25 ft lbs. Make sure the muzzle device is square against the shoulder. When setting the silencer on the muzzle device ensure that the threads and tapers are clean. Torque the suppressor to 10 ft lbs. Ensure alignment. Enjoy. The nitride finish does lead to a tighter seal with that said it is extremely important to torque and rockset your muzzle device to the barrel properly.