I shot a friend's 8 ga double rifle off a bench at Pachmayrs years ago. It was more of a big shove than any kind of pain. Kind of like being suddenly shoved by a train.
I sighted in some of the big Weatherby rifles for people, and I definitely remember the pain. A Westley Richards 425 with a stock that was excellently designed to transfer energy directly to your shoulder felt about the same. Those were what I would imagine getting hit on the shoulder by a ball pein hammer would feel like.
No need to go through that again.
