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400 NE,
Yes, you are right about the oiled cartridges although who knows that the proof-house folks didn't have some measured data around oiled cathridges?

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Of course they did. The original standard "pressures" (these were axial measurements of bolt thrust, not chamber pressure) of British rifle cartridges were standardized in base crusher guns with oiled cartridges. Proof is conducted with dry cartridges. Absent the high pressure proof cartridges, oiling a standard cartridge dramatically increases bolt thrust, but can do nothing to test radial integrity. Only high chamber pressure can do that, and high pressure "purple stripe" proof cartridges have been used from day one. Modern standards for these cartridges are piezo electric transducer measurement of chamber pressure (which is measured radially), and proof loads must produce a mean pressure of 125% of the maximum average operating pressure.

You mentioned that you were looking for a shotgun action for a DR conversion that would adequately handle 45,000 PSI. I would suggest some careful reflection about that. Krieghoff's factory ammunition for their .500/.416, one of the highest pressure flanged full nitros, has an average pressure of 44,962 PSI, and CIP max average is 45,686 PSI. That's a touch high for a purpose-built flanged nitro double rifle. Holland's famous bolster to their back-action sidelock Royal, a dedicated double rifle action to begin with, was inspired by the high pressure of their .375 Flanged Magnum - 47,137 PSI. The idea of building a .500/.416 on a shotgun action makes me shudder. Keep in mind that a proper proof load for a 45,000 PSI operating load will be 56,250 PSI.

If you must use a shotgun action, it would be best to keep it to Nitro for Black/BPE pressures.

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"Serious rifles have two barrels, everything else just burns gunpowder."

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