My gunstore commando quote wasn't directed at you, and I apologize if it appeared that way. It was not my intent to belittle your opinion or you in anyway.
My intent was a general suggestion for people not to listen to others opinions so much as it was to look at the facts presented before them before they made up their minds. Opinions they should listen to and give some weight would be the various experts out there and what they recommend. I doubt you will ever find a big name firearms instructor that recommends birdshot for home defense. You're the first firearm instructor I've EVER heard say that. Out of curiousity, what round does your department issue you for your shotungs?
http://www.backwoodshome.com/articles2/ayoob031221.html an article by massad ayoob about shotguns, including a section on ammo selection. A quote from that article when talking about buckshot "These pellets will normally stay inside the assailant’s body with a front to back shot, reducing danger to innocent bystanders who might be located unseen behind a violent criminal. "
For those of you who don't know, Massad Ayoob is a LEO, a LEO trainer, a former lawyer and prosecutor as well as the head of the LEthal Force Institute.
Ballistic gelatin simulates synthetic soft tissue. Muscle, organs, etc. Bone is a much harder and more difficult for projetciles to pentrate. Hence the relatively large penetration requirements for the FBI (18"). 6" in flesh isn't the same as 6" through the chest as the bones will deflect and stop a lot of the pellets.
Anectdotal evidence isn't data and I would caution it's use as such. I've got anectdotal evidence that says that birdshot isn't as effective.
I've also seen shotgun wounds first hand. Both in people and an animals. A hunting accident where a hunter was crossing a fence, dropped his shotgun and shot himself in the chest. The person who was shot walked out of the woods holding a rag to his chest, and drove himself to the hospital, 25 minutes away. We were hunting squirrels, so I'm guessing he probably had #6 shot in his 12 gauge.
By the same token, I could also bring up the story of Trooper coates, it was brought to my attention in my first NRA instructor school. Tropper coates was killed in the line of duty by some scumball with a .22 pistol. This was after trooper Coates put 9 .357 magnum rounds into the guys chest. The guy is currently serving a life sentence in prison (he survived the 5 357's to the chest, but trooper coates was killed by a single 22 round to the side).
The fact is, that nothing you can shoot by hand is guaranteed to drop a person without putting it in the right spot. Going light and thinking it's going to be as effective as something heavier doesn't make a lot of sense. That's like saying a .22 is just a dangerous as a .357 at close range, so I'm going to carry it instead.
It is an interesting topic, and a fun discussion.