Note that #8 birdshot penetrates about 4 inches. Which can kill you, but is not exactly an ideal defense load.
Sorry, but lethality often isn't the only criterion for a home defense round.
Never said it was. Personally, I want:
A) maximum possibility of rapid / instant incpacitation
B) Adequate, but not excessive penetration
For me, that's #4 buck. Penetration falls in the same range that the FBI protocol requires of pistol ammunition (12-16 inches), but does not have the excessive penetration that 00 buck and slugs have (22 and 30 inches respectively. Great if you have to shoot a guy behind a sumo
) Twenty-seven pellets also gives a pretty good number of wound tracks, for bleeding, and more chances to hit the CNS than 00 buck's 8 or 9 pellets.
CNS disruption or blood pressure drop sufficient to cause unconciousness are the only physiological means of incapacitation. Fear, pain, the psychological shock of being shot, etc can't be counted on. CNS disruption will cause incapacitation pretty much instantly. Blood loss could take a little longer, and a few seconds is long enough to get beaten to death / shot.
Don't get me wrong. I completely agree that birdshot can be very lethal, particularly up close. Would I want to get shot with it? No. Would I use it for self defense if that's all I had? Yes. Would I still use it if I had my choice of ammo? No.
That's all I'm saying. Yeah, it would have been a lot faster to just say "I agree," but I feel wordy today.