The two least pleasant firearms I've ever shot were the tiny scandium snub nose revolvers by Smith and Wesson (the second one because I couldn't believe the first one hurt as much as I remembered-it did. :laughing

and a stupid single shot 20 gauge Stevens.
The revolvers hurt because, well, they weigh virtually nothing, and I was given some of the IMI 170 grain ammunition to try in them. Foolish and dangerous; I didn't know that's what ammunition they had. My hands don't fit the weapon properly, so I got wrapped on the knuckles by the trigger guard to boot. I see the utility and appeal, but getting through a basic qualification course with normal ammunition would be awful.
I don't know why the little 20 gauge hurts so much. Somehow it manages to hurt the shoulder and the cheek bone more than any firearm I've ever shot. I have a fairly light 375 H&H that I do pretty good work with, and it doesn't hurt anywhere near as much as the single shot. Nor does my Wild West guns 457 mag or any of the military weapons with metal butt plates. It feels like receiving the blunt end of a hatchet on the shoulder and the face each time I shoot it.