RM,
Some people think there are legitimate reasons to use birdshot for HD, and I am in no position to argue with them. I have zipped body bags on a couple of folks who were killed by birdshot at close range in the years I worked as an EMT, so I know that birdshot can in fact be lethal.
I also know one person who survived a point blank shooting with buckshot as well. This young man was a classmate while I was in school, by the time he got out of the hospital and up and around again, he was a year behnd me. He was extraordinarily lucky to be alive after his hunting accident despite permanent damage to his right leg- he lost about 3" out of his femur.
The simple truth is that nothing involving defensive firearms is 100% certain, what you want to do is try to skew the odds in your favor in every way possible. Keep in mind that you are not interested in killing, whether your assailant dies or not is not your main concern. Your intent is to stop your adversary from doing whatever it was that was sufficiently serious to cause you to have to shoot him. You need to bring about that stop as rapidly as is humanly possible, before your assailant has a chance to cause physical harm to you or to your loved ones Since rounds from a shotgun as a rule do more damage than any other projectile that can be launched from a firearm that one person can easily handle alone, shotguns tend to be popular as defensive firearms. But muzzle blast or proximity of passage of the projectiles in question won't necessarily stop an attack. You have to get hits on your assailant to do that.
With any firearm projectile intended for self defense, you have two primary considerations- PLACEMENT and PENETRATION. You have to deliver projectiles to a place on the human target where important structures are located, and said projectiles have to be able to penetrate sufficiently to disrupt those structures. That's pretty much it in a nutshell- placement and penetration. Placement is easier with a long gun than a handgun, so I prefer shotguns. Penetration depends on your choice of ammunition in large part.
I personally don't use birdshot in defensive shotguns, because it cannot be relied upon to penetrate deeply in human adversaries. In fact I even stopped using reduced recoil buckshot recently, after hearing some troubling reports of lack of penetration in shootings with reduced recoil buckshot loads. I now use full velocity loads of 00 buckshot (Hornady TAP red hull, to be specific) in the magazines of our 'house guns', with Kent/Brenneke KO slugs riding in the SideSaddles.
I am in no way telling you or anyone else that you should do what I do, naturally- you have to make up your own mind what to use for defense based on your own particular situation and circumstances. I base my decisions regarding these questions on the best information I can get, and on experience with the firearms and loads in question. Of course I have no magic wands or magic bullets, there are no guarantees I or my wife can produce instant stops on determined, drugged and/or demented assailants with the rounds I have chosen. But I believe they skew the odds in our favor as much as possible. And that's the best I can do from that standpoint. The rest comes with delivering those rounds as accurately and quickly as possible, which takes training and practice. Those two things are IMO of greater importance than mental gyrations regarding selection of defensive firearms or ammunition.
Stay safe,
lpl/nc