More than you really wanted to know...
jmorris said:
So again, it’s simply a reference point, change the diameter and you change the point, by how much depends on the taper.
If you are making relative measurements you can pick any diameter you want, to be the “Datum point”.
This is actually not correct. You can pick any arbitrary location on the case shoulder to use as a reference to make a before and after resizing comparison to determine how far the shoulder is set back, but the fact this arbitrarily chosen diameter is a reference does not make it a datum.
As I mentioned before, there are several datums on a cartridge and chamber (marked with a "B" for Basic on the SAAMI drawings), and these are chosen by the cartridge designer or by agreement among SAAMI members. Their purpose is not just to define headspace. They are the dimensions that are the takeoff points for other dimensions that define the shapes of the cartridge and chamber; the taper of the sides and the locations of the intersections of the neck and shoulder. They allow you to accurately determine the dimensions of cartridge case forming tools and to set up a precision grinder to sharpen a chamber reamer with the right profile. An arbitrarily chosen reference point, by definition, does not provide that information so it is not a datum.
There is a complication that occurs resizing older rimless cartridges precisely that can be spotted in their case and chamber drawings. The .30-03 derivatives the 30-06 and 270 Winchester drawings, for example, show a difference between the case and chamber shoulder angles such that the angle of the case shoulder is slightly greater than the chamber shoulder angle is. This is because headspace used to be figured not to a shoulder datum (that's a SAAMI innovation; the CIP doesn't use it) but from the breech face to the intersection of the shoulder and case body. If you have a copy of Hatcher's Notebook it shows this as the headspace on the .30-06, and they wanted to be able to determine where the outer perimeter of the case shoulder before it touched the rest of the chamber shoulder because that's what defined excess headspace.
A result of that is that if you take a random reference point too close to the neck and try to find shoulder setback directly, you can actually set it back a thousandth at that location and not have set the outer edge of the shoulder back at all. This is because the resizing die is made to the case shoulder angle while a fireformed case is set to the chamber shoulder angle. Unfortunately, the difference in shoulder angles is one thing in SAAMI's drawing and another in the Garand blueprints and I've seen a couple of other old drawings suggesting still other angle pairings have been used. So your best bet for an unfamiliar chamber is to measure the case to whatever shoulder reference you please, then mark the case shoulder with a Magic Marker and set up your resizing die, slowly turning it in while checking the ink for contact marks to determine when shoulder contact is complete. Then you can take a measurement to the reference again to see how much it changes before shoulder setback actually commences.
This all assumes, of course, that you want to be that precise. Most won't care, which is why I titled this post as I did.