boomtube, I have to disagree with a couple of things. For background I reload for over 40 rifles of all makes and calibers and measure shoulder position with a Hornady Headspace Gauge (although it should be called a shoulder gauge)
you said that
"Any fired bottle neck case has expanded to fit the chamber in both diameter and length (to the shoulder)"
IME, it takes several firings for most cases to expand to fit the chamber. A typical set of measurements if you measure on the shoulder and neck size only would be (this on a 30-06)
new case - 2.040"
once fired - 2.0485" (then neck sized)
twice fired - 2.050" (then neck sized)
3 times fired - 2.051" (slight crush fit, neck sized)
4 times fired - 2.0515" (crush fit, neck sized then body sized)
Now this is a little bit too much headspace but the gun shoots extremely well and the cases create no problems.
"Fact is, real world permanant expansion and reduction of brass cases just isn't as precise as some seem to think possible. Small varitions of the alloy within individual cases and the amount of work hardening each case has experienced will typically cause shoulder length variations of as much as 3 thou, occasionally as much as 5 thou. When I read of people claiming to set shoulders back exactly .001", etc, I cringe because it simply isn't possible to be that precise."
I have set my die and sized as many as 50 cases and had variation of shoulder position in the range of .001" or even less. I do keep my cases marked and sorted by how many firings they have been through so the work hardening is the same.
Now belted magnum cases vary greatly in the measurement on new cases vs once fired case expansion. This is a post 64 Winchester 264 win mag new case and once fired
that is .037" movement of the shoulder on the first firing! It would probably have wound up at close to .040" but I bought it for rebarreling anyway. But the headspace on that rifle before rebarreling was .006".