I'm not sure I would be too impresssed with a smith who didn't remove the barrel for this procedure unless he had lots of experience and could convince me it would be done properly or he would fix it, quickly.
Bob
Of course you have to take the barrel off. I am just saying that this is not an AI where you do the .004" setback. There is no reason to touch the threads or tenon unless you want to index the barrel a certain way that is different than it now is. Simply run the reamer in until the case you plan to use fits. You can't go by the gage because they both headspace off the belt. Both use the same gage. You could have the reamer maker grind you a go-gage that goes off the shoulder if you want. I generally use a piece of factory brass that I have stored in my die box for setting up 300 Wby rechambers.
You can also mark the belt cut with ink and sneak up on it. Bottom line is that you don't need to 'set' anything back.
