Roy
I believe that is a non issue as well. If removing around .140 inch diameter from a typical 1.200 Sporter Shank causes the bore to change any measurable amount, I would like to see how you are measuring it.
I rough the tenon out to within .005, then establish as good of a chamber as I can in the barrel. I then perform all of the other machining operations.
The reason I rough the tenon out is that is the only time that there is any appreciable tool pressure to possibly disturb the set-up.
Incidentally, after I establish the chamber, I check it and the throat area with the .0001 indicator to insure that it is as it should be..........jackie