I have seen the figures for chamber pressure of the lowly .22LR rimfire, but do not have them handy now. I seem to recall they were at least approaching the 20K psi range. This is far above the the max allowable SAAMI pressures for even a modern 3½" 12ga Mag, yet I do not recall anyone ever mentioning "Shooting a gun Loose" from using one of the .22LR insert bbls fitted to a shotgun bbl, nor splitting a stock. My original post may not have been "Technically" correct, but in laymans terms I do believe it to be 100% correct. To figure the strength of a tube the important consideraation is pressure. To calculate frame strength, stock strength etc the important consideration is "Back Thrust" what ever terms you want to call it. Most looseness in break open shotguns is the result of frictional wear from improper maintenance & lubrication.
While certainly pressure is a contributing factor the "Back Thrust" is a combination of the pressure over the area it pushes against & the factor seldom mentioned of time.
I can fire a 2½" .410 with ½oz of shot @ 1200 fps & attain 11K psi. I can fire a 12ga 3" with 1½oz shot @ 1315 fps with 10.7K psi both using Hercules/Alliant 2400 powder. Assume both of these loads fired from guns having same stocking, same strength of frame etc, etc & I do not by the most elaborate of imagination believe I really have to ask anyone here which load would put the most strain on that gun. If you want to determine what load will come the nearest to "Shooting a Gun Loose" calculating the recoil of its load will be far more meaningful than will its chamber pressure. Incidently both of the above mentioned example loads were taken from an Alliant Manual & used Rem Hulls & Rem Wads. I hand-picked them for "Close" pressures but with any difference being for higher in the smaller .410.
I could just as easily point to 2 3/4" 12ga loads of 1oz/1200fps vs 1¼oz/1330fps; both @ an identical 9,500 psi. the lighter load simply "Will Not" "Shoot the Gun Loose" as quickly as will the heavier one even though both have an identical chamber pressure.
"I Rest My Case"
Do understand of course, the chamber pressue must be within the limits of what the chamber walls can take. I would certainly not recommend shooting an olg gun with thin chamber walls with ammo having pressures approaching SAAMI standards regsrdless of how light the recoil. If we look to extremes with a fast enough powder bursting pressures could be reached while producing almost no recoil as the pressure could rise so fast virtually no movement of the shot would have taken place while the pressure escaped out that big Gaping hole in the chamber side. The stock would of course be intact.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra