This has been quite a little whirlwind here...
I can understand the choice to use birdshot in a close, heavily peopled living space, but it is a very compromised defense load, suitable only when necessary by circumstances. Otherwise, it doesn't make sense.
I'm no expert, but from what I know, I would say that a hot 12 ga. birdshot load at 7 feet or so to COM of a moderately built man (NOT wearing a heavy leather coat) would probably take out the bad guy right now... it's certainly quite capable of that under "just the right circumstances".
On the other hand, at twice that distance or more and given a heavy leather coat, winter clothing, bodybuilder or obese type of guy, there's no way I'd even think of trying to rely on birdshot. I can't imagine how Harley would want to RELY on that either, when the evidence is overwhelmingly in favor of buckshot when the barriers get thicker and the distance longer.
So Harley loads up for defense use with birdshot expecting to be up against people who aren't heavily clothed or built and they appear inside his bedroom at less than 10' range. Chances are probably decent that his birdshot may stop the guy OK.
BUT, what does Harley do if he has to go outside to try and protect someone else who is being harmed by the big bad guy and the distance is 15' to 25' and the bad guy is big, heavy, cracked up, leather clad, whatever? The birdshot he expected to work well at 7' inside his bedroom is likely to fail miserably in stopping the perp under these difference circumstances.
On the other hand, if he had loaded with 12 ga. buckshot to begin with, it would be supremely effective at 7' inside the bedroom, and still very effective at 17' down the hall, and also very effective at 27'+ in the yard against little guys and big guys, lightly or heavily dressed, sober or all hopped up.
The buckshot is simply a more reliable defense load under a wide range of circumstances. I can't see why a reasonable person doesn't understand the wisdom in choosing buckshot for defense. It just makes the most sense because you never know what circumstances you will find yourself up against. And if I knew that behind the perp was a room containing an innocent person, I'd be as disinclined to shoot birdshot as anything else. A high collateral damage-risk background is just that, no matter what the weapon or load.
Signed,
another "12 ga. buckshot kind-of-guy".