A 12 GA recoil can hurt! My Rem 870 loaded with 1 OZ 00 buck or same weight slug had recoil, but the light weight 06 hurt more.
This is the olde kinetic energy formula E = ((m*v)^2)/2
where E is foot pounds,
m = mass in slugs (gun weight in pounds/acceleration of gravity giving slugs), acceleration of gravity = 32.163 fps/fps,
v = gun velocity in feet per second
^2 indicates values squared
/ Indicates division
The gun velocity is shown by:
The gun velocity is calculated by:
Vg = ((bullet wt. * bullet velocity) +(powder wt. * gas velocity))/(gun weight in pounds * 7000)
This calculates gun velocity by dividing total momentum by gun weight in grains.
Finally, the kinetic energy is determined by using gun velocity & other.
Just guessing that gas velocities would be less with lower pressure 12GA loads than 06 loads.
Of interest, at one time had a 12 pound 6.5-06 with a 14X, 1 1/2-inch Unertl scope. The scope was about 18 inches long and had external adjustments, like the entire scope moved vs. reticle image inside scope. The scope also had an external recoil spring that returned the scope to position after the shot. Upon making the shot I could feel the spring compress then decompress as it pushed the scope back. The sort of heavy steel tube scope with spring was acting like a recoil buffer. The recoil impulse was delayed making felt recoil softer.