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It is of questionable value...esp. when used with anything other than new brass. As frass is used, it become harder (less spring back), can anneal the necks...but the HEADS get harder too, and that's what we are measureing.

At best, it can give an aproximation of when a handload is at the same pressure level as a factory load. Need to sacrifice a box of factory shells.

Select 10 rounds and break them down...what we are after are the primed cases. Put the primed cases away for a time.

Mark the LOADED cases 1 -10, measure each of loaded 10 rounds at the is a blade mic. as we want to measure the SOLID head just ahead of the rim. Record the numbers. Fire the rounds and re-measure for expansion. Record the numbers. Find the average case head expansion.

With the 10 cases you broke down, reload with your selected load. Go through the unfired/fired measure again. IF the expansion is higher than the factory load, can assume the pressure is as well. IF the expasnion is less, can assume the pressure is also less.

I gave up trying to eek out the last few FPS and load a bit conservativly. My thninking is that if I have to run a round to the ragged edge of safety, I really need a more powerful rifle.
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