I know this is just an anecdote that doesn't tell us much about the stopping power of shotguns but my oldest cousin told me a story while he was interning at a hospital in Memphis back in the 1980s.
A homeless fellow was brought in that had been hit in the lower abdomen with a shotgun blast. I don't know the gauge or the shot size but I was told the Xray showed almost 100 pellets that were stopped by the skin on the fellow's back after passing through from the front. My cousin said he worked for twelve hours on the fellow before the man finally died.
It seems like there was a Canadian study of birdshot penetration that considered sizes and distances and it wasn't impressive. I don't think any came close to the recommended 12 inch minimum.