I did a thread correction for one of George's customers, it was the most shocking muzzle thread i have ever seen.
Rifle was a very nice Sauer Outback in 308 that had been threaded by a "reputable" shop.
This so called 1/2x20 UNF was .484" diameter, did not have an undercut at all, was torn and had no chamfer on the muzzle end.
The moderator would not butt up against the shoulder but the customers previous mod (T8) went on no problem!!
All i could do was chase the threads form an undercut and make sure the shoulder was perpendicular to the bore, ideally the muzzle should have been re-threaded but due to the fluted barrel, re-threading would have cut into the flutes, something the customer did not want.
I had a photo but my PC crashed last weekend and i have lost all my previous files.
Ian