caliper on its own is not going to give you a good shoulder measurement. look at the hornady comparator body and headspace bushings. they fit onto a caliper and let you get a decent measurement on the shoulder datum.
Workshop Innovation - Gunsmithing & Reloading Tools: Reloading Stuff +, Gauges
If a "factory spec" case, i.e. something that was full length resized in a die that was set up correctly does not chamber, you may want to get a gunsmith to have a look at the headspacing. It only takes a couple of thou for a case to be too large to close the bolt, but it may also be something more serious....