Most threads are designed to mate with 75% thread, but some guns I have seen are around 80%+, and are tight when threaded up. If you remove material, the holding power of the thread drops dramatically.
If you are fitting a new barrel, you always work off the bore. First, you center the barrel in the lathe, off the bore, turn the thread boss on the end, and then thread it. Then when you bore and thread the frame for center, the bores center line should match the bolts center line. You measure the threads on the barrel with thread wires, and calculate the cut depth with them.
This does not take long on a lathe, as after you finish the barrel to spec, you bore the receiver, and cut the threads until the barrel will screw into it, with the desired tightness you want. In other words, you use the barrels threads as a gauge for the receiver. Last, to get the sight on top, you face off the barrel shoulder, until the barrel threads on to the correct position. You can generally calculate how much to face off by knowing the threads per inch, and using where the barrel sits when tight, say a quarter turn until it's there, you remove enough off the shoulder face for almost a quarter turn. Last, you need to remember that you might have to face off the rear too, because of headspacing. The bolt has to close against the barrel, and the chamber length needs to be correct too.
Last edited by Dixie Gunsmithing; April 28, 2013 at 08:16 AM.