That's an interesting example, but does it really relate to double rifles? I'm thinking the water tower displays a "steady state" pressure which represents the pressure of gravity pushing the fluid through varying diameters of pipe. Whereas with a rifle, you have an explosive force, not gravity, and then we must look at the actual forces involved. A 7mm Mauser is about 45,000 cup (if I've found the right information) whereas a 30-40 is about 39,000. There isn't any appreciable difference in the size of the "pipe". A 416 Rigby generates less pressure in cup than a 240 Weatherby, again having nothing to do with diameter. But does this mean you'd have to build a DR in 240 Weatherby HEAVIER than one in 416? I don't know the answer to that, and I sincerely doubt it by the way.

I'm no engineer, and as Huck Finn says "This is too many for me..." but I think the better question is the one posted above by Huvius. What exactly takes the force of the explosion? Certainly part of it is the chamber walls themselves. Part of it pushes the bullet out. Part of it is backward against the standing breech. But how do all these forces play on the action? Does the hinge pin take most of it? Does a Greener crossbolt do anything in real life? Is the Doll's Head bite merely window dressing? I don't think I've ever seen these questions dealt with anywhere. I suppose Holland or Rigby at some point (one would assume?) purposely blew up a double (perish the thought) to see where it "broke". These mechanisms don't come from pure imagination. But when converting a shotgun action, it becomes really important to know where your weaknesses are.

And this doesn't take into account a lubricated case, and the backward thrust that would put on the mechanism. A heavy bolt action is an immensely stronger mechanism, and even they can be strained by this form of testing.

Like I said, "It's too many for me." But, absent any real numbers or destructive testing, I think that we should be able to look at original rifles, and go with the same basic dimensions (thickness of chamber walls, hinge pin diameters, etc.) and with modern steels, we'll be just fine. Unless someone wants to donate a dozen converted shotguns for a painful day of blow-up tests, this is probably as close as we'll come.

"If it looks right, chances are it is right." This has built a lot of palaces and cathedrals and bridges through man's history.

Sorry for the long post.