Good heavens, I just re-read my reply , sorry chaps it was very late at night and I did not spell check it,. What terrible English,
It should have read...

Metalguy

It is not the pressure per se that is the important factor. You need to know the MAP for the two cartridges and the base diameter that the pressure works on in the two cases.

so R1xR1xPyexMAP=breach thrust.

Now you need to compare that to the thrust for 12 gauge cartridge that was the original chambering.

What you need to bear in mind is that the action is going to try and bend around the root where the standing breach meets the watertable.

If you want to do the calcs properly one should also consider the moment of the force as a smaller diameter acts lower down the standing breach and reduces the force acting on the root.

The hoop pressure i.e. 47,000 v 42,000 is relatively unimportant as the barrels steel (if the gun is properly assembled will happily take twice that pressure.