With many of the loads I develop I find that primer flow around the firing pin is the first indicator that I am nearing Max Pressure. To measure this I use my calipers and zero them on the length of a fired case. I then punch the primer and measure the difference which is primer flow. In my 20 EXTREME there is Zero primer flow with 39/40 grain bullets running at 3500 fps with my low pressure gopher loads. Push them up to 3650 and I get measurable PRE and primer flow which I like to keep at less than 0.003" which is less than some factory rounds in 223 and 204R. Primer flow can reach as much as 0.008", without piercing, but there will be CHE and perhaps even blown primers at these pressures if the bolt does not get sticky.
Primer flow is not necessarily a function of pressure,it is usually more related to mechanics of your particular bolt and chamber set up , pm me and I can explain , we run over 70,000 and have no measurable primer flow or signs of over pressure . All has to do with proper and exact fit of brass , dies and chamber
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