There are several recoil calculators on the web.
As others have stated, recoil is dependent only on ejecta mass and velocity. Note that there is no place to input peak pressure as it is irrelevant.
This is an extreme example that illustrates why one shouldn't merely guess at recoil. Some would be quick to assume that the recoil in a 6 lb SxS would be more severe with a 1 &1/4 ounce load than with a 1 ounce load and often that would be the case. But not necessarily. Since both velocity and mass are the major factors, it's necessary to know both. Pushed fast enough, a 1 ounce load can recil more heavily than a 1 & 1/4 ounce load....
I like the above calculator (
http://www.handloads.com/calc/recoil.asp )because it lets me compare 2 loads side-by-side. The numbers I used are from data published in Lyman's 3rd edition.
The foot-lbs of recoil energy begin to make sense when I correlate the input values to real-world situations. I know that I can tolerate 300+ target loads in a day, provide they are 1 ounce @ 1250fps. I also know that 1 ounce @1300 fps would be objectionable. In my 8.5 lb O/U, that's a difference of only 2 ft/lbs, 14 versus 16.
I know I can shoot 200+ in a day with my 6.5lb 20ga SxS with no ill effects. Not surprisingly, 7/8 ounce @1175 generates....14 ft/lbs.
I know that 1 &1/4 ounce @ 1260fps is about the most I want shoot through my 12ga, 7.5lb 12ga bird gun. That's 25 ft/lbs. No wonder then that when I use my 6.5 lb 20ga for late winter pheasants I'm OK with a second barrel load of 1 &1/8 ounce of #5 @ 1175fps...that's only 22 ft/lbs.
