Industry Std on threads. When cutting threads on a barrel the threading tool can not reach completely to the shoulder. at the end of the threads should be a relief cut a deeper cut that causes a void in the threads.
Now some manuf of silencers put a relief cut in the silencer so it does not need a relief cut on the barrel, some do not, some barrel threaders put a relief cut, some do not.
In your case a flat washer is going to do the trick as it provided a way to not run out of threads before it squares up.
Also measure the exposed threads on your rifle barrel, and the thread depth in the mount on the silencer, if the threads are short on the barrel than the threads in the silencer not filled by barrel will gunk up with blast carbon, if the thread on the barrel is long, the exposed thread inside the silencer can gunk up with carbon and make it hard to impossible to screw the silencer off.
When having barrels threaded have it done by a gunsmith who mounts silencers, not just one who puts muzzle breaks on for hunters.