Originally Posted by SidecarFlip
It is possible to gage the 'dent' in the primer itself, however it takes some doing..... You need to take a small (1/16" or less) diameter ground ball and mike it to be sure of the od.
Measure a like unfired primer for it's overall (height dimension).
Place the ball in the 'dent' and measure the combined height dimension and subtract the height dimension of the primer and subtract the dimension of the ball. What's left is the depth of the 'dent'.
A bit involved but it will give you the depth of the dent.
I've heard this before. Never worked. And it varies with firing pin strength. Popping a primed case without bullet and powder results in a deeper dent than a live round with normal pressures.
Get a gage to measure primer protrusion, then measure it with a micrometer or caliper. For near full shrouded bolt faces (Rem 7XX and post '64 push feed Win. 70, for example) use a depth micrometer/caliper with the bolt removed.
Or, with a brass rod equal to barrel length, use a depth micrometer/caliper to measure firing pin protrusion from the bolt face.