Quote Originally Posted by cswaffie View Post

Also, how important was the shoulder seating against the shoulder in the chamber when dealing with head space? I always thought head space was measured at the rim.
On a 303 Rimmed cartridge the shoulder does not seat against the chamber 'shoulder'. The cartridge rim seats in the rear of the chamber.

The headspace is measured with headspace gauges that simulate the rim of the cartridge.
The cartridge rim specification is 0.064" max and 0.054" minimum.
The headspace gauge is 0.064" minimum (the same as the max cartridge rim dimension) and 0.074" maximum (which allows for 'wear & tear')

If you have a 0.064" thick cartridge rim and a 0.074" headspace then there is an additional 0.010 (10 thou) 'sloppiness', if however you are using some of the American made brass which tends to be on the thin side you could have a rim thickness of 0.057" and with a headspace of 0.074" have an additional 'slop factor' of 0.017" (17 thou)

It is this 'slop' that we try to tighten up by using larger (longer) bolt heads and reduce the EFFECTIVE headspace - however if you adjust your headspace to minimum for Rem, Win brass etc (example 0.057" rim) and then try and use Military surplus, HXP or Prvi cases (0.060"-0.064") you will struggle to close your bolt due to the increased rim thickness.