So a quick update... i received my comparator and took some measurements. I was really surprised at the results.
Lapua Cases for .223
new=1.413
fired=1.419
resized=1.408
IMI Cases for .223
fired=1.415
Bottom line is that with my resizing die configured to contact the case holder, headspace of the resized brass is shorter than fresh new brass. WOW! i would not have expected that.
I did an experiment and FL resize the LP brass to 1.419 and test fit in my chamber. There was slight resistance when closing the bolt and after removing my brass, i rechecked headspace and it read 1.417. I did not expect that so i repeated the experiment again and had the same results. So now i realize what the actual headspace of my chamber is.
Going forward, i am setting my headspace to .001 less than my chamber... which is 1.416.
This was a great exercise.
One problem with the Redding solution, you need a new set of shellholders for each case type.
This note is a variation on the competition shellholder idea:
Buy
these shims. They cost $15 from Brownells.
Using your standard shellholder, set up your die so it contacts the shellholder. Lock the lock ring in place. Using a new Lapua case and your comparator, verify that headspace is 1.408 -- as above.
You fixed the lock ring on the die. Unscrew the die while keeping the lock ring fixed in position on the die. Position the .010 shim between the press and the lock ring, screw the die back into the press. Size another case. It should measure 1.418. That is, with the ten thou shim the case should be ten thou longer than without the shim and one thou shorter than the fired case length.
Suppose you want headspace to be two thou shorter than fired length, use a .009 shim or use a .004 and .005 shim. Want three thou shorter? Use the .008 shim. Less shim means more headspace.
Suppose that you are sizing IMI cases, find the right shim for IMI and use that.
If you anneal after each firing, the shim required to get a specific headspace will stay the same. If you don't anneal, the brass gets harder and you need a different setting in order to get the same headspace. This technique makes it easy to adjust in .001 increments without having to re-setup your die.
Things that can go wrong ... If you size a lot of cases, the die could work loose and the headspace will get longer. [solution: put a witness mark on the lock ring and on the press. While sizing, periodically check the marks. This problem exists whether or not you use shims] The lock ring gets loose, then there is no telling what happens to headspace. [solution: before each use, make sure the lock ring set screw is tight].
Those are the big issues. There are a few other things like getting lube on the shims or lock ring (lube has dimension), getting crud on the shims or lock ring, having two or more presses.
This works for me. Not every solution works for every person.