Brass flow into the ejector and measuring expansion just above the extractor groove tells you how hard the brass is and its elastic spring back limits.

Meaning softer brass will flow into the ejector hole at a lower pressure and this tells you it time to back off no matter the actual chamber pressure.

One problem with measuring expansion above the extractor groove is the thickness of the flash hole web will add strength to the base of the case.

The left case below with the thinner flash hole web will show more base expansion and loose primer pockets before the case on the right.




Below the Remington case will show brass flow into the ejector before the Lake City case.

How Hard is Your Brass? 5.56 and .223 Rem Base Hardness Tests

http://bulletin.accurateshooter.com/2014/05/how-hard-is-your-brass-5-56-and-223-rem-base-hardness-tests/