here we go again
We kick this around so much, and I am guilty myself of loading a coach gun w/ bird shot a time or two for SD in very tight quarters, like a motel room. Very narrow niche and I have rethought the matter since.
The std OO buck load has been the yardstick for a long time and serves much better as a GP/SD load for shotguns, and both my HD shotguns are so loaded at present as well as the duty gun in the gov. vehicle. I live and work in the boonies.
What might be a better idea in the "niche" of extreme urban dwellings and protecting oneself at room distance is something like BB or T-shot (T-buck) ........and that is just a notion w/ no proof.
A gelatin study with 00, BB, T and #4B, and say #6 shot, at reasonable ranges, to include barriers like common clothing and jackets would be very interesting.