Back to the OP's original question -- presuming a bolt action.
Originally Posted by OP
...goal right now is to determine head space and correctly adjust my dies. I use RCBS standard dies
is the distance forward from the bolt face to (about) midway on the shoulder portion of the chamber. It's usually "X" inches, and manufacturers are allowed ± a few thousandsths leeway.
is the distance measured similarly -- from the base of the case
to that same midpoint on the case shoulder. It's usually "Y" inches.
If Case 'Y' exactly equals
the Chamber 'X' dimension, you have a perfect headspace match. But normally the case should be
a few thousandths shorter/smaller to ensure reliable bolt closure.
How to precisely determine/set up the sizing die?
Use EITHER the Hornady Headspace Comparator set, ...OR the RCBS precsion Mic.
It makes no difference.
1. Fire a few rounds. Assuming the fired cases will actually go back into the rifle and the bolt close on them, carefully measure/average the resulting case
headspace dimension. That is your DEFAULT
case headspace number.
2. Screw the sizing die into the press and down firmly to the shellholder. Then back
it off one full turn.
3. Using plenty of case lube, size one of the fired cases. Because the sides are being squeezed in, the case length (and headspace dimension) will likely grow.
4. See if the case will chamber in the rifle. If the bolt will close with no
feel, measure the new (probably longer) case headspace and write it down as the new #2_DEFAULT case dimension.
5. If the case will not
chamber, gradually screw the sizing die down (1/16 at a time) until the bolt will close with just the barest
of feel. Measure the case headspace again as the MAX case dimension.
6. Size several other cases with this die setup to make sure the bolt will just barely close on all of them. Measure them all and write down the AVERAGE MAX.
7. Subtract 0.002" clearance from the AVERAGE MAX and set the die up (lock the lock ring down) every time to produce cases with that same dimension: (AveMAX - 0.002")
8. Check the first several cases in each loading session (making sure you've used a good/standard amount of lube) to ensure you are producing cases with this dimension. Adjust as required since brass springback/stiffness can effect things a thousandth or two between firings.
* Yeah, I know there's supposedly no such thing. But that's too bad. It helps explain/work in the real world.