"PRIMER PROTRUSION" head clearance measurement tests/conclusion

WHY I DON"T MEASURE HEAD CLEARANCE USING THE PRIMER PROTRUSION METHOD
Here's how I used to do it. I don't use this method now.
1. Use fired, FL sized cases.
2. Measure the length of the case with a vernier, dial or electronic caliper.
3. Write down the measurement.
4. Start a primer into the case.
5. Remove the bolt.
6. Put the case extractor cut under the extractor-this holds the case on to the bolt.
7. Put the bolt in the action and close the bolt gently.
7. Extract the case carefully .
8. Measure the length of the case and protruding primer, from the primer to the case mouth, with the caliper.
9. Write down the measurement and subtract the first measurement from the second. The difference is the amount of primer protrusion which is the head clearance.
10. This measurement, as with all measurments, is best done a number of times and the results averaged.

Why don't I use this method?
Some testing showed that no matter how carefully this measurement method was used, the measurements, compared with measurements using an RCBS Precision Mic, were remarkably and unacceptably inconsistant.
(Here we define CASE HEADSPACE as typified by the SAAMI 308 Win cartridge dimension 1.634-.007 on the SAAMI 308 Win drawing at http://www.saami.org/pubresources/cc...Winchester.pdf )
The RCBS Precision Mic measures CASE HEADSPACE as the deviation from CHAMBER HEADSPACE minimum dimension. For example, SAAMI 308 Win minimum chamber headspace dimension is 1.630". A cartridge case with the equivalent dimension of 1.632" would measure +2 using the RCBS Precision Mic. 1.625" measures -5.

First test 6/11/12:
Measure the case headspace of 30 308 Win cases with an RCBS Precision Mic. Measure the length of each case. Start a primer in the primer pocket of each case and chamber it in a M10 Savage. Measured the length of the case from primer to case mouth. Subtract one length from the other, getting the difference = primer protrusion..

Case # Primer
Headspace Protrusion
(.001") (.001")

-6 3 3, 3, 7
-5 5 2, 3, 4, 5, 5
-4 19 0, 2(4), 3(4), 4(4), 5(3), 7(2), 8
-3 3 2, 4, 7

Second test 6/16/12:
Same procedure as above with 12 223Rem cases.

Case # Primer
Headspace Protrusion
(.001") (.001")


-2 10 2, 4, 4, 7(5), 8, 8
-1 1 7
0 1 2

Primer Protrusion = Chamber Headspace - Case Headspace.
For example, with chamber headspace of 1.632" and case headspace of 1.629" 1.632"-1.629" = .003"; the expected primer protrusion.
We would expect that Primer Protrusion measurements would track Case Headspace measurements within +/-.001" or so; yet the variation is all over the map..
It is clear that the results of this testing show that Primer Protrusion does not vary as the Case Headspace, and for my purposes this measuring method is unacceptable.