Wayne,
I think we are on the same page!
Pressure certainly creates the energy to accelerate the ejecta but it alone is NOT a factor in determining the amount of recoil any load will produce in a firearm.
The attachment is from SAAMI for the Free Recoil Energy formula. It is teh most accurate and is pretty complicated. For our purposed here is an easier to use version from another forum:
Free recoil=
(weight of the bullet X muzzle velocity + 4700 X weight of powder charge)squared
__________________________________________________ ___________________
64.348 X weight of gun in pounds
The top line is divided by the bottom line. Sorry about the poor graphics above.
To simplify it even more this is a link to a recoil calculator:
http://kwk.us/recoil.html
Pressure creates movement of the ejecta against its launch platform which in our case in a shotgun. The movement of that platform is the recoil based on the total weight of the eject and it's velocity, both of which are countered by the weight of the gun. The heavier the launch platform the less recoil energy. Hmmm, in theory the less movement in the launch platform the greater the velocity of the ejecta although in most cases this would be darn hard to measure
Excessive recoil may crack our 100+ year old stocks, especially if a stock is soaked with decades of cleaning oil...
Excessive pressure will loosen an action of it too excessive produce a bulge in, or even burst a barrel.
The attachment is the Gun Recoil Formula from SAAMI: