Adco Firearms, well known for their AR-15 gunsmithing services, recently share the image below along with the following words of caution for dealing with nitride finished barrels…
Black Nitride is a good surface hardening technique, but it also makes things brittle. When timing a flash hider to a 1/2×28 threaded 9mm barrel, don’t use a breaker bar to try an get one more rotation. It will snap off like glass.
This is probably a good reason to use a timing shim set instead of a crush washer on nitride finished barrels. The timing shims will allow you to time the muzzle device without putting as much stress on the barrel which will prevent the above and have less negative effect on accuracy.