The only real way I would see to completely answer this question would be if an action could be fitted with a gauge of some sort to measure its deflection when fired. I do not have the capability of doing this nor do most others here.
Consider a few points though which have been determined over the years. Total barrel time from when the primer ignited until the load exits the barrel on most normal shotgun loads is on the order of .003 of a second. during that time the gun itself will have moved no more than about 3/8". The rest of the guns movement in recoil comes from the momentum built up while the load was traveling down the barrel plus a small percentage due to the gas exiting the muzzle.
As the pressure has peaked & is starting to fall before the load has cleared the chamber then it is at max pressure for an Extremely short time. As pressure is applied to the standing breech it takes a certain amount of time for deflection to occur. How much longer does 6K psi have to be applied to equal the stress on the action as 8K for a shorter duration.
Until shown absolute proof to the contrary I am still of the opinion that weight time velocity stresses the action & stock, PSI stresses the chamber walls.
1Ľ oz @ 1300 FPS is about 20% higher than 1 1/8 oz @ 1200 FPS.
Within the confines of what will produce an acceptable load for a shotgun pressure itself does not enter into this equation.
As the OD of most shotgun barrels have more taper than does the chamber itself even as to the matter of stressing the chamber walls the higher pressure will occur where the walls are thicker than that of the lower pressure load so the difference is not as much as many suppose.
As of yet I have been shown no really Good & Valid reason for loading much below the 8K level. Most powders burn better at that level & you don't have to worry as much about the possibility of a squib if you shoot it in cold weather.
As to the recoil I guess I'm just not "Blessed" with the sensitivity of some. If for instance I load some shells with an ounce of shot to 1200 FPS I truly cannot tell you whether they are loaded with Red Dot, Green Dot or Unique, even though they have differing burn rates thus differing rates of acceleration. Up the velocity to 1290 FPS as are many of the factory promo loads which have to be able to function a semi-auto & yes I can Feel that.
