I asked the same question just a couple of days ago. The general consensus was .001," measured as follows:
1. Take a once-fired case
2. Using a .0001" blade style Mic, measure the diameter of the extractor groove.
3. Load up and shoot that same case, them measure it again.
However, a guy I know is a professional ballistician. He has written a book on reloading. He used an Oehler 43 PBL for all his pressure testing. He found many interesting things, like some Rem brass would go to 80,000 psi before showing ANY signs of pressure. When I asked him why he didn't show a correlation between case head expansion and pressure, here was his answer:
"The reason I did not show a correlation between web measurement and pressure is because I have not been able to find any. The general rule of thumb is 0.0005" of web expansion in a once-fired case indicates pressure around 60,000 psi. At a measured 60,000 psi, I have had as little as no measurable expansion to as much as 0.0007". On the average, I have not really seen any measurable web expansion of a once-fired case at less than 65,000 psi.................
I am sure you will notice, I said a once-fired case. A new case will give widely varying web expansion measurements, and a case that has been fired more than three times will become work hardened -- which will also distort the pressure/expansion correlation......................I am sure you will also note the measuring precision I indicated. It is necessary to have a micrometer accurate to 1/10,000 of an inch to do any meaningful measurements across the web. It is also necessary to have a blade micrometer because the round anvil of standard micrometers is too large to fit between the rim and the expansion ring. (Actually when I measure web diameter, I measure across the extraction groove rather than in front of it. This tends to allow me more consistent, repeatable readings.)
I know all of this hasn't been much help in giving you some guidelines for estimating pressure. The best suggestion I can provide is to fire-form a few new cases, measure the expansion ring, partially resize, and load them as you desire. Measure the expansion ring of the fired reloaded cases, and if the expansion across the expansion ring is over 0.001", reduce the load a little."
Blaine
