Re: Case Base Expansion as High Pressure Sign
Measure within 1/8" of the extractor groove. The big thing is be consistent. How much is a very dangerous question because of all the variables in a chamber.
Best thing is to shoot factory ammo with the same brand of brass you are using. Better is to sample a few and compare.
Measure what the expansion is. Compare to your load. This only works for new virgin brass. Measuring expansion on once or multi fired brass will give the wrong results.
Also, compare to muzzle velocity. If more then 100fps above listed max vel, back off. Then there is primer flattening, etc, etc.