OK, but a top lever on an L.C. Smith which is left of center, or even somewhat worn and at center, would not tend to rotate to the left when the gun suddenly decelerated during recoil against the shooters shoulder. And the top lever spring in a Smith gun is not tensioned in line with the bore axis. I see your point, but that would not explain an L.C. Smith or Ithaca N.I.D. opening upon firing, but camming of the rotary bolt would.
I agree that it would be interesting to see some high speed video of what actually happens at the moment of firing under various conditions and with various makes of guns. I'd also like to see a video of someone holding the breech closed on an unbolted gun with their thumb and forefinger during firing with a full charge as Greener suggests. But I am not volunteering my thumb and forefinger, having been peppered in the forehead with gasses by pierced primers, and once being totally blinded for about 20-30 minutes after a complete head separation on a .22-250 cartridge. Ouch!
