Pistol champ.

With respect. The limiting factor for most doubles is breach thrust not absolute pressure itself although one is proportional to the other and case diamiter. There is no reason to expect a correctly built double either at home of by a major manufacturer to be unsafe with the rounds mentioned. If we take the case of the 44 marlin ( I have one) the barrel shank diamiter is less than the one on my double and thats not a guess I've rebarreled one so hoop stress is not an issue. The breach thrust @ 44,000psi of that 44 Mag is 7213 Lbs. The breach thrust of my donder 12 gauge in its origional chambering which is proofed at 1370 bar is 10,453 lbs.

I agree I would not fire a built double by anybody until it had passed a proof testand indeed the very first round fired in mine was done at the London proof house under controlled conditions.

That home built double is now proofed to 3500 bar and a breach thrust of 13109 lbs, double that of a 44 mag.

As to the 375 H&H, that high pressure is the reason that there is a 375 H&H Flanged for double guns that runs at a lower pressure.

Again with respect mine is a "real" double rifle. Many of what are regarded as classics now were made in London in workshops not much bigger than mine on trade shotgun actions purchased in the white. My double shoots 2" off hand groups at 50 mts with full out hunting loads. That is as real as it gets.
No I wouldent be happier with a purchased, cheep gun (I can afford one). I will be delighted with my first hunt with the gun I built. I was like a scrapyard dog with two cocks when it regulated to 1" at 25 mts, it is a part of me in the way that nobody elses product could ever be. It is unique, a phone call from my firearms department confirmed that today as they couldent enter it on the London polices computer, as they don't have a paramiter for it, as nobody else in London has one in that old caliber. I cant buy that.

Regards