I'm going to agree with Miller that there is more than just chamber pressure at work when a gun is fired. The kinetic energy from recoil due to the mass and velocity of the entire ejecta accelerating down the barrels can hardly be ignored. There is also torque and vibrations to consider in that split second after the primer detonates.
I will also defer to Ted's intimate experience with the design deficiencies of the Tobin guns. I believe him when he says the frames are weak due to large deep milling cuts. In the past, I've mentioned the obvious fact that some guns stand the test of time much better than others. There are a number of makes of doubles that are very seldom seen in a sound, tight, on face state. Some of these guns were made in and sold cheaply in extremely large numbers. Yet few survive, and when you see them now, most are either parted out or quite loose and worn. I don't believe this is because they were shot any more than higher quality guns. If anything, these were guns used by poorer working class folks who were less prone to waste ammunition on silly things like clay targets.
A lot of that is poor design. But I think both Ted and Miller might be overlooking another possible reason that Tobins bend and stretch aside from deep cuts in the frame or high pressure and heavy recoiling loads. I would suspect that part of the problem is in the steel used for the frames. Miller spent a lifetime milling, turning, and machining steel and other metals. I'm sure he would agree that steel is not all the same. It is an alloy of iron and other metals and carbon in varying percentages. There are thousands of different recipes or alloys with very different properties. And how that steel was processed also has a large bearing on how it performs. Drop forging, heat treating, annealing, etc. also enter into it. I wouldn't be at all surprised if the steel used in the frames for Tobins and many other guns that didn't stand the test of time was simply a lower grade or inferior quality product.
