^ uh...
Wouldn't it just be easier to see the laws of physics demostrated in something like a test media?
Brassfetcher's test on #1 Buck - penetrating to about 13"
http://www.youtube.com/watch?v=cz0g0...5&feature=plcp
Brasfetcher's test with #4 Buck penetrating to about 8.5" (I'm just guessing based on how far apart his braces are on the underside of the block.
http://www.youtube.com/watch?v=PDjNaTeUt2Y
My main points are
Shot spreads out and shot slows down, it does this in air and it does it in water or ordnance gel or the human body.
You can visibly see that the #4 shot slows down much more quickly than the #1 Buck, it penetrates less, but also, the direction of some of the #4 Buck changes at a greater angle than the #1 Buck.
Just the laws of nature - try changing the direction of a rolling marble versus a rolling shot put. Try stopping a rolling shotput versus stopping a rolling marble.
Birdshot does spread out, and it basically comes to halt.
I'm thinking that shooting someone with birdshot in the throat / neck is probably more lethal than shoting them in the chest. The throat is relatively soft tissue, major arteries are only a few inches deep, but even if the birdshot didn't penetrate to a major blood vessel, tearing up the esophagus and the trachea is going to cause a person to choke to death.