Ed, the only relation that pressure has to recoil is the fact that without any chamber pressure, there will be no velocity of the ejecta. Recoil is the result of Kinetic energy produced by the load we fire in our guns.. Google the formula for calculating Kinetic energy, and you will see that pressure is not any part of that calculation.
In the calculation for recoil energy, the only variable components are mass and velocity. (I am neglecting the slight rearward jet effect of expelled gasses at the muzzle) The mass and velocity of the gun moving in the opposite direction of the shot, wad, and burned powder is what we call recoil, and that force, measured in ft./lbs. is kinetic energy. Other factors enter into "felt" or "perceived" recoil, such as stock design, buttplate area, etc. But chamber pressure is never a part of these calculations.
In other words, it does not matter if the peak chamber pressure that propels say a 1 1/4 ounce load to 1200 ft. per sec. was 5000, 10,000, or 15,000 psi. The recoil of that identical shot charge at that identical velocity in the same gun will be essentially equal. Of course, there will be a greater mass of the total ejecta if you propel that shot load with 3 1/2 drams of 2Fg black powder versus a much lighter charge of modern smokeless shotgun powder. Different powders and different components will produce different pressure curves, which explains why very different peak chamber pressures may produce the same velocity with the same shot charge. It isn't peak chamber pressure that determines muzzle velocity. It is the overall average pressure produced by the entire duration of the powder burning within the barrel.
And this is why certain powders with different burning rates are known to be best for creating loads that provide useful velocities at lower peak pressures. This is why we complain when powders like PB are discontinued. These are less likely to create excessive strain on Damascus barrels, or other mechanical components that may not withstand the pounding created by modern factory loads. Old stock wood tends to become more brittle or subject to damage with time and degradation from things like oil soaking, or becoming loose due to shrinkage of the bedding contact surfaces, etc. Using loads which produce lower velocity and contain lighter shot charges reduces the recoil which might otherwise damage old stocks.
You are not the only one who has confusion and misconceptions about this subject. It has been discussed literally thousands of times here and on other firearms forums. I hope this helps to explain why peak chamber pressure has virtually nothing to do with recoil energy calculations, and is not the sole reason we seek out lower pressure loads and lower velocity loads for our vintage doubles.
Some here will complain and insult you over your notions about pressure versus recoil. But those common misconceptions are not nearly as stupid as those of any gun owner who supports anti-gun Liberal Left Democrats. Some people need to learn to prioritize about who is more of a threat to the future of our gun rights.
