Chris, I’m certain that my 12ga 3 1/2” Mossberg 835 Ultimate mag - recoils far more then my .375 H&H - loads i’ve Shot (and been bruised) include 12ga 31/2” 21/4oz Turkey loads and 31/2” slugs. My Mossberg weighs approximately 71/2lbs, is pump action and with 31/2” loads is the hardest kicking shotgun I’ve ever fired - exceeds my .375 H&H and just below my .470 NE double rifle. It is so unpleasant to shoot that I’ve limited it to 23/4” & 3” shells and use only as a “back up shotgun” in the event one of my others breaks or gets stolen..... I’ve killed deer with it using 3” 00 buck (knocks deer off their feet inside 30 yards) but never tried the 31/2” buckshot loads - because I’d likely be “Off My Feet” and spitting out teeth!
For all that recoil the 3 or 31/2” shells have no more killing power then 23/4”....just more pellets and denser pattern - not worth the punishment to my shoulder or jaw. Regarding slugs, any 12 ga 1 Oz or 1 1/4 oz Slug that’s “accurate” will create the same size hole, wound channel, penetration - they all act like a “Beer Can” going through a deer....plenty of killing power. However, I would never consider hunting Cape Buffalo or any DG with a Shotgun Slug —- they are Not equivalent to a .375 H&H and will not penetrate as well. I have minimal experience shooting African DG but extensive (over 75+) deer with shotgun using buckshot & slugs. Given the choice I’d always pick a good rifle.