I've finally come up with a comparison for those people who seem to think that birdshot is a magical home defense round.
The myth goes that because buckshot "overpenetrates," that one should use birdshot, so that if you miss, it won't "overpenetrate" the walls and hurt others. After all, it's just as massive so it's just as good, right?
If that were the case, we wouldn't have "birdshot" and "buckshot," we'd just have "Shot."
Now, there are any number of tests on gelatin and people that prove this is wrong. Why people refuse to believe them, I don't know.
The facts are that any round that will reliably stop a hostile WILL penetrate a wall, the end.  And that in combat or self defense, all those niceties of range safety disappear.  Troops in combat do sweep each other with loaded weapons, do have fingers on triggers, do shoot near or past each other, and do shoot without necessarily knowing what the backstop is. One tries, but one is also trying to, you know, STAY ALIVE.
A shot column acts like a fluid.  The entire mass isn't interacting at once. The leading pellets impact, and shed velocity first, then act as buffers for the following shot (if we're assuming close enough range for it to impact all approximately at once).  At a distance, each pellet alone lacks the power to do much of anything to anything bigger than a bird. It can, in fact, be even LESS effective up close than it otherwise would be, due to the buffering effect.
Let's use this comparison:
If I throw a snowball at you, it breaks up when it hits.  I can hit you harder if I throw it faster, or if I use a bigger snowball. But within the capabilities of human throwing, the snowball is going to be annoying and messy.
If I freeze a ball of solid ice of the same mass and heave it at you, it can smash your face to jelly.
It's the same mass. It's also a single mass.
Birdshot is for birds.
