DRJ,
Both Italy and France are members of CIP to the best of my knowledge, and would use CIP rules.
CIP requires proof loads for rifles to be 125% of the max service load pressure.

A maker, an individual, or a company "could" ask for a higher percent proof load if they wanted, but I sincerely doubt that any break action double rifle would ever be subjected to a proof load of 7450 bar. Both the Verney Carron and the Sabatti would be built strong for sure, but I can't see too many double rifles surviving proof loads producing 108,053 psi!

The CIP requirement for a proof load for the .458 Win Mag is 5375 bar (77957 psi). This is the service load pressure of 4300 bar multiplied by 125%.

Jack, I think some of your bar to psi (and other) calculations are off.
You can check here.
bar-psi psi-bar