I gotta add this one, the Cheney Shotgun Experiment...
http://www.myscienceproject.org/shooting.html
Ballistics gel is actually fairly expensive, especially if you were just going to use it to test this out. There are a ton of resources online about this (for example, the Box ‘o Truth http://www.theboxotruth.com). Another site has some good real-world information here: http://www.thehighroad.org/showthread.php?t=381023.
It’s interesting to note that many of the posts seem to think that’s inadequate. If you ask me, 5.5” of penetration and what looks like at least 2” entry wound is going to stop any human on this planet. People always like to bring up the example of a mythical fat guy wearing a gigantic heavy jacket, but I don’t think that’s going to have much of an effect. The pellets do their damage both by entering the attacker and by bouncing; those that do not break bones will often bounce (doing even more damage). As many posts around will tell you, birdshot can definitely be fatal and buckshot can be less than lethal. While there is no doubt that buckshot is more damaging, for things like price, availability, and safety, I believe birdshot is the better option.
I will note that many objections are based on the FBI’s rather odd recommendation of at least 12 inches of penetration (and preferably 18) for a particular round or shell. In the vast majority of cases, 12 inches of penetration will put the projectile squarely through the bad guy and out the other side.
Still, in any case, it’s relatively cheap to take a thawed ham or chicken out to the range, toss some layers of clothing or an old jacket on it, and see what happens at various distances. I have never heard of a single person who was shot with birdshot and continued what they were doing before. Some might live, but I’m pretty sure it ruined their day.