Interesting thread.
I had an old converted 'Banermanized" 16 guage black powder caplock shotgun for a while. It was quite capable of delivering punishing recoil, when loaded with 100 grains of black powder, and a healthy dose of shot. I think I was using 12 OO buck. The shot was deadly too, at least on small trees. Easily penetrated 2.5" poplar never got the chance to use it on game before I traded it.
It hit the shooter about like a stout 12 guage smokeless load would, and with a much heavier gun than a modern shotgun.
You are of course talking black power RIFLE Vs Shotgun, and here I think that the black powder rifle has less felt recoil than a typical 12 guage, with slug loads. However, there are exceptions. Some autoloaders really do soak up a lot of recoil.