I get that brass has different hardness. This was done between Hornady brass on the 300, and norma brass on the 28. I was more curious how web expansion can dictate pressure since i had 9 thou expansion with no sticky bolt. Then have 2.5 on a tighter cut chamber and have sticky bolt.....
I have a really good article that was in Precision Shooting photocopied and tucked in the pages of one of my dozens of loading manuals, but could not find it last night.
In the mean time this is a short discussion from Hodgdon's website:
http://www.hodgdonreloading.com/reloading-education/tips-and-tricks/simple-trick-monitoring-pressure-your-rifle-reloadsUsing a blade micrometer that measures in ten thousandths (.0001"), new, unfired cases can be gauged before and after firing to determine reasonably accurate maximum loads. Micrometers measuring in thousandths (.001") are insufficiently accurate to perform these measurements, and should not be used.
Previously fired cases cannot be used accurately due to various levels of brass hardening. Measurement is taken just ahead of the extractor groove on the case head and must be taken at the same place on the case before and after firing. By placing a small mark on the case head – entering the cartridge in the chamber with mark at 12 o’clock – a consistently accurate measurement can be taken with each firing.