Your Shoulder & the Gunstock does not Know or Care what the pressure is.
What gets to the stock & shoulder is how much shot you throw & how fast you throw it.
If you throw 1 1/8 oz @ 1200 fps no matter if the pressure is 5K or 10K you have done the same amount of work. "Average" pressure for the entire length of the barrel is pretty much the same. Perhaps not absolutely identical, but not enough difference to Get up on your Tricycle about.
DO NOT confuse peak pressure with total pressure. When you change burning rates of powder & wind up with identical ballistics you have simply re-arranged the pressure curve, not truly reduced it
The shot clears the barrel in a matter of micro-seconds & the gun will move in most cases no more than about 3/8 inch. The rest is from the inertia built in during that brief moment of time.
It Is a proven fact that low peak pressures can, & indeed have, given the results mentioned. You can believe it or not but it has been well documented. It occurs more often in Rifles than Shotguns, but can occur in shotguns as well.
Smokeless powders for the most part just don't burn well below a certain pressure level. When you get below that level then you are treading on Thin Ice.
