I just recalled an incident from my mis-spent youth, and will have to revise my answer in post #4.
I was somewhere between 10 and 12, at my uncle's farm, and for some reason the adults were gone. My cousin and I decided to shoot my uncle's single shot, break open 12 gauge. If I recall correctly it was either a winchester or savage and weighed about 5.5 pounds. My cousin said that he thought the 12 gauge really kicked. (All we had ever fired were .22's and a .410). I was up first and thinking I was wise, I hitched the hard plastic 12 gauge butt plate tightly my shoulder, and then backed my shoulder up, hard against a stout tree.
All I can say is that I was probably lucky not to break my collar bone.