There's a REASON that bird shot is called "bird shot."
There's a REASON that buck shot is called "buck shot."
Birds are tiny little animals with thin skin and thin, fragile, hollow bones.
BUCKS on the other hand are about the size of a human, and skin about the same thickness and strength. Additionally, their bones are surprisingly similar to ours in density and toughness, especially their ribs.
In hunting birds, one wishes to kill the bird in-flight without blowing it into pink mist. A few tiny pellets are usually sufficient, and a larger pattern makes it easier to hit flying birds.
Hunting BUCKS on the other hand requires much more penetration - it must not only go through skin and bones but must penetrate deeply enough to damage arteries or the spine - thus stopping the animal from running away.
A wounded buck that escapes and dies later of his wounds or MUCH LATER of infection is no good - we need to drop him where he stands if possible - turn him off like a light-switch.
It should be obvious by now that HUMANS are much more like BUCKS, and bear little resemblance to BIRDS.
If your opponent is built like a bird, I guess birdshot would be an option - but since he's built like a Buck, buck-shot or slugs are the **ONLY** viable option!
Those who think that an ounce of lead is an ounce of lead are mistaken or deluded.
THINK.
Even at HD distances, with minimal spread, you're still shooting little tiny things with birdshot - and they just don't penetrate like buck. There are countless examples of people who were shot point-blank with birdshot and lived. The odds are much, much worse for anyone hit with buck!
If you value the things you're supposedly protecting - YOUR life and those of your FAMILY - USE BUCK!
If you value the PERP'S life above those, then feel free to use birdshot! Why take a chance on really hurting him~!