thought this material might shed some light on this for you......and on another website, I had one guy come back with the missing material regarding recoil velocity on the 12 ga 3" and 3.5"...... when matching the two necessary components "energy" and "velocity", you get the two issues at hand...."the push" and the "sharpness".....turns out that if you can manage a 12 gauge 3.5" mag for turkey or geese you are right in the zone of the felt recoil seen with a .458 Win Mag....not sure if this helps you, but most forget the measured comparisons of other shooting arms that we tend to just shoot and accept....
ok....think I found my answer.....went to Chuck Hawks tables for both rifle and shotgun.....here is what I found...
listings included 12 ga 3" and 10 ga 3.5".... and the rifles included a significant aray of calibers.....without a 12 ga. 3.5" listed I had to interpolate an answer based on the provided data.... recoil was identified as energy in foot pounds for both shotguns and rifles....rifle weight was also identified.....data breakdown is as follows....
12 ga. 3" was through a 8.75 lb. gun shooting "1 7/8 oz" of shot at 1210 MV ---- recoil energy is 54 ft. lbs
10 ga. 3.5" through a 10.5 lb gun shooting "2 1/4 oz" at 1210 MV, also ----- recoil energy listed as 63 lbs
I would guess that the 12 ga 3.5 would be at least toward 60 lbs, if not slightly over
now the interesting part: expressing the rifle weight and ft lbs. of energy yields some interesting comparisons
.300 Wby Mag - 150 gr @ 3400 MV through 9.25 lbs of gun weight ---- recoil energy is 24.6 lbs
.340 Wby Mag - 200 gr @3100 MV through 10 lbs of gun ----- recoil energy of 29.6 lbs
.375 H&H Mag - 270 gr @ 2690 MV through 9 lbs of gun ----- recoil energy of 36.1 lbs
.378 Wby Mag - 300 gr @ 2900 MV through 10.25 lbs of gun ------ recoil energy of 71.1 lbs
.416 Rigby - 400 gr @ 2400 MV through 10 lbs of gun ---- recoil energy of 58.1 lbs
.458 Win. Mag - 500 gr @ 2100 MV through 9 lbs of gun ---- recoil energy of 62.3 lbs
.460 Wby Mag - 500 gr @ 2600 MV through 11.25 lbs of gun ----- recoil energy of 99.6 lbs
These are very interesting data points for those who have issues with putting a heavy caliber big bore to shoulder but at the same time will put a big 12 ga. load to shoulder on a bench when we pattern our shotguns for spring turkey.....I for one put a new BPS 12 ga NWTF 3.5 to my shoulder this way a couple of weeks ago to get ready for spring gobbler....patterned the turkey full choke at 15, 25, 35 and 45 yards for two different federal 3.5" load types....had a nice case of recoil rash on the shoulder....was enough to prompt taking it to the gunsmith to have a new kickeez installed to replace the factory pad.....but all of this was instructive to me....I have never ventured past a Wby .300 Mag in centerfire....and the notion of putting a Brown Bear load up, not to mention a load for African Big Game, was something that just never held much attraction....so these comparisons are interesting and informative....hope they are to you as well