Grenadier.

You cannot proove an action, only a barrelled action.

The proof pressure of the 12G doner action is irrelivent, it is limited by the chamber wall thickness not the lug sheer strength.
As Marrakai pointed out the thrust is the important figure.

If somebody insists in converting a Ruger O/U which is not a great shotgun base to begin with, into a double using .375H+H which was never designed to be fired in a double rifle then any problems could be likley anticipated.
But .444 on a good S/S action will not move in a lifetime. The thrust bearing area is so much greater than a lever action rifle for which the cartridge was designed.

Regards

(edited because I was being a grumpy old prick

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