All other factors remaining constant such as the chamber/gun, type of hull, and primer, when pressures continue to raise a primer will eventually be pushed against the flat wall of the breach.
I have often seen primers from my trapshooting friends' reloads that have been pushed flat against the breach face. Their loads often exceed 1300 PFS for 1 and 1/8 oz. Now, the pressure of those rounds may have been within the 11,500 SAAMI 12 gauge working maximum but they are far above what I desire to shoot in a Damascus gun!
If you load the same hulls and primers at 6000 psi and also 10,000 psi take a look at the edge of the primer cups where it curves from the flat portion that the firing pin/hammer strikes. The primers from higher pressure loads will start to loose their curve. When pressure is higher (not necessarily too high) that portion of the primer will fill in.
I have noticed a slight flattening of primers with my hunting loads. This is not referenced to any measurement of pressure but it does indicate that something is happening!
Here is a photo of a flattened primer from a rifle.
There is a lot of debate about this subject on the reloading and rifle and pistol shooting forums. My experiance from loading for both rifle and pistol is that when I pushed to the top of the loading data the primers flattened out upon firing. Also, when the bolt was hard to open the primer was very, very flat!
PS: If I opened my Parker and noticed primers as flat as in the photo I would drop me my knees and thank God for sparing me from my stupidity!
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