This one of the best and easiest to use recoil calculators I have found. Additionally, it does a fair job of explaining free recoil calculations.
http://www.omahamarian.org/trap/shotshellenergy.html
You can easily play around with different parameters to see the effect they have on free recoil. You will quickly see that shot charge, velocity, and gun weight are the parameters that have the most effect with shot charge being the most significant. You will also see that chamber pressure has no bearing on any of the calculations.
Hybrid Mode