What thread sizes get used on 6.8 and 300×221 barrels?
As you probably know, 1/2″-28 is the standard thread size used on 5.56 barrels. Many muzzle accessories are available with this thread spec, but some of them will not clear a 6.8 or 30 caliber bullet. Some combinations of bore size and thread size can leave a very thin barrel wall at the threads. I prefer to have at least 0.100″ barrel wall thickness at this junction. It’s possible to stretch the barrel at this point by torqueing the muzzle device into place. The proper way to install a muzzle device on a barrel with a thin wall would be using loctite and very little torque. I can open up the muzzle device for bullet clearance and also open up the thread size to a larger size which leaves a stronger connection to the barrel. See my services page for details. Below you will find a table with common thread sizes, common bore sizes the the resulting wall thickness at the threads. It is my opinion that 1/2″-28 threads should NOT be used on barrels with a bore larger than 6mm. Thread Spec:
