Mr. Pritchard,
May I suggest that Mr. Camp is right.
Buckshot is round. The worst possible (practical) shape for aerodynamics is a round ball. They don't slice through air well at all.
Given that they lack range in air (00 buck good to about 30yds max) they will lack overpenetrative qualities in meat also.
Knowing that, while the human body is not homogenous, it is 70-90% water, it's safe to assume that the shot will stop as water is much denser than air.
The Foster slug was invented for two reasons: Accuracy and weight. Until the point of the Foster, people were shooting round, .72 caliber lead balls from their 12 gauges, effectively turning the combination into an earlier type of firearm: The Musket. This was bad for both accuracy and penetration. You'll get an idea if you ever look at the old tests conducted by the US Army for adoption of the rifle over the musket. (As an interesting aside however, this is what Wyatt Earp used to hunt buffalo as he could get in close and shoot with reasonable accuracy. It save money over a Sharps or similar "buffalo gun.")
The Brenneke slug was another answer to the same problem; it uses a semi-wadcutter profile and an attached wad to stabilize the projectile.
Here's a summation of the common defensive projectiles used in 12 gauge shotguns:
00 Buck: Nine to 18 .32cal round lead balls. Not typically overpenetrative but the threat still exists at close range. In live game (Indiana whitetail) I've seen 00 buck vary widely. When used correctly (<25yds, heart/lung) the deer ran maybe 50yds before collapsing and bleeding out. When used incorrectly (long shots, bad shots) the deer was lost. 00 buck does not suffer fools or bad placement.
000 Buck: Varying amount of .38cal round lead balls. Perhaps a bit more penetrative than 00, but not typically enough for concern. I would not say the increase in individual pellet size and weight offsets the reduced ballistic payload over 00 buck. That said, I've not seen 000 buck in action. However, I would believe that the performance would be similar to 00 buck.
Foster Slugs: In deer, personally observed, Foster slugs expand flat about 50%-75% of the time and anchor deer with good hits about 25% of the time. Reports coming back to me indicate that the deer are almost never anchored on the spot and that the slug always expands. "Never" and "always" are viewed by me with a jaundiced eye.
Brenneke Slugs: These suckers are my preference. Though they do not expand they are of a semi-wadcutter profile and leave a larger wound channel from the get-go than do Fosters with their round noses. Typically penetrative, I've not recovered one, and must assume little to no expansion. I've not personally seen one fail to anchor a deer with a proper shot to the shoulder/heart area. There are those who still believe in hitting the lungs however and that takes the time you might expect for blood induced pneumonia. In other words, around an hour on average.
All that said...
My personal loading preference for my shotty is 2-3/4" 00 buck, 9 pellets, from an open cylinder.
I have this backed by Brenneke slugs for +25yd shooting.
I don't mess with birdshot. However, X-rays of victims indicate that birdshot acts much like a large frangible bullet at close range (<10ft): Shallow penetration, massive localized tissue damage with many separate tiny wound tracks. At a measured 25yds both #6 and #7-1/2 shot will stick in 1/4 inch plywood with several bouncing off. It appears to me that it could be a viable SD round at distances where its wad is retained and effectively enters the target as one projectile which comes apart (again, a large frangible).
In my opinion, the 12 gauge shotgun, properly loaded with either 00 buck or Brenneke slugs and shot by a practiced operator, is the best defensive civilian weapon to be had at distances in which the given ammuntion performs best. That said, if I were pressed to choose just one type, it would be the Brenneke slug as it performs from bad breath to ~100yds.
All that said: There are records of people taking hits from the .50BMG and staying alive long enough to fight.
Josh <><