How is a manufacturer supposed to "proof" a self loading unmentionable with a double charged round? You're just risking damage to the firearm.
I have just spoken to a very pleasant young lady at the Birmingham Proof House (44-121-643-3860) who advises me that unmentionable self-loading items ARE in fact proofed with a substantial overload, and if they fail, they fail. Needless to say, these days the failure rate of modern-manufactured firearms ex-factory is passing small - so small, in fact, that I've never ever heard of it happening.
It is usually older guns that are required to be proofed so that the owner can shoot them. Y'see, here in yUK, you CAN have antique guns of any kind, especially muzzle loaders, and those that would use an obsolete cartridge, WITHOUT ANY KIND of permit. If however, you want a rifle or carbine that fires a currently available cartridge, like a Trapdoor Springfield in .45-70 Govt, then that is a whole new nest of frogs, and it needs a yUK Firearms Certificate [FAC] for you to own and shoot. AND gun club membership, AND a whole scad of other stuff....
However, going back to older guns per se, IF you want to shoot it/them, or sell it/them to somebody else who DOES want it to shoot in vintage arms comps, then it MUST be proofed first - that is the law. It must then be registered on the owner/new owner's Firearms Certificate as a live-firing Section 1 firearm, if rifled, or entered on the shotgun certificate [a Section 2 firearm] if a smoothbore with barrels over 24" long. Less than that, and it 'becomes' a Section 1 firearm.
Of course, these days in yUK, only those lucky beggars domiciled in Northern Ireland are allowed to have modern handguns as a matter of course. Here on the mainland, we CAN have the so-called long-barrelled handguns, and ANY BP muzzle-loading handguns, but ownership of cartridge-firing handguns is VERY restricted indeed. And not a subject for discussion on this forum.