Shotgunlover, I agree that if the toplever, and the bolt, cammed open enough to completely clear the bolting surfaces, the gun would possibly begin to open and the bolt would not be able to go back home until the barrels were returned home.
You saw how much the top lever moved, and you know that would not be enough movement to actually totally unlock and open the gun. If the forces of firing could be sustained for more than mere milliseconds, the barrels would not rebound back and could continue to open the breech. But since the impulse is very short, and the bolt has not totally opened, the top lever spring certainly could return the bolt to full lock position just as it would upon normally closing the gun.
As Miller suggested, some guns have more tapered bolting surfaces than others. If Jones, Purdey, Holland, etc., ever developed a self opening gun that allowed itself to be popped open upon firing, they would immediately scrap that design and return to the drawing board. Guns that don't stand the test of time and guns that slowly self destruct end up getting scrapped. That's why we don't see too many loose Foxes or L.C. Smiths, and why we don't see too many tight Crescents or JABC's, even though they made a lot more Crescents and Crescent variants.
I cannot imagine any inertial force generated upon firing that would rotate a top lever to the right, can you? But there are all kinds of forces, vibrations, harmonics, etc. happening in that split second after the primer detonates. Hanging my pickup truck off the muzzles would certainly bend my barrels, but would not duplicate those firing forces. If there actually was such a left to right top lever rotating force, firearms designers could capitalize on it and design a gun that actually bolted tighter upon firing. I don't pretend to know the absolutely correct answer, but Miller's explanation of a tendency for a tapered rotary or under bolt to be partially cammed open upon firing still makes sense to me. I would love to see an absolute and irrefutable answer, and I'm sure that either you, Miller, or myself would be reasonable enough to accept whatever that turned out to be. Until then, it makes for interesting discussion.
