Shotguns are not proved to those pressures because:

The cartridges do not generate those sort of NE pressures and thus it is unnecessary.

The barrel wall thickness of a 12,10 etc gauge will not tollerate those pressures. Thus you cannot prove at those pressures as the barrels will fail long before the action lets go.

This does not mean that the action itself and the lugs and any third bite will not tollerate those pressures.

Some high price hand made shotguns are still made on low carbon case hardenable steels. From a conversation the other day with somebody who should know, EN 32 & EN 34. Most mass produced shotguns are 4140 or 4130. Thus and older or high quality shotgun may have issues with the increased pressures without some form of designed in reenforcment (bolsters etc) However it is likley that a modern mass produced shotgun is machine made of tougher steel in the first instence and is perversly better equiped to handle the pressures.

For this reason for my first project I choose a shotgun designed for 3 1/2" Mag 12 gauge cartridges which was designed at the factory for the simultanious discharge of both barrels. It is more than man enough for the job. IMHO is is stronger than a 100 year old English DR of unknown history that we would all be proud to shoot without reservations.

It has now had several hundereds of rounds through it, it has been shot by a PH who is a sponser here and there are no issues with it. It is still as tight as a nuns chuff.

Biakial for instence use the same single shot action and same double action as their shotguns for their rifles. These are chambered for amoungst others 30-06 at much higher pressures than we are discussing. They are proved as all european products must be.

IMHO it is a mistake to look at the classic rifles and assume that the same ammount of material is necessary nowdays, it is not so. Metalergy is so advanced since the last war that it is apples and oranges. Look at cars, you used to need a 305 CU" engin to get 250 BHP, you can get that now from a 2000cc all alloy engine for a third of the weight. Cars used to be heaps of rust in 6 years the steel was rubbish compared to that which we work with now. Look how small the bearing surfaces are on an M16/Ar15 bolt but good enough for thousands of rounds of sustained fire. Polymer frames on glocks with bearing rails.

I am not advocating conversions for the reckless but I see no reason that they are fundermentally a bad idea.

Regards

PS after I wrote this I thought that I would look up the numbers.

EN 32 after carburizing has a tensile strength of 430 N/mm square.(62,373 psi)
4140 has a tensile strength of 1000N/mm square @ HRC 29 ( 145,000 psi)
4130 860 (124,746 psi)