Harley:
Your test(s) don't accurately replicate the penetration of various shotgun loads into a human wearing clothing for various reasons including:
1. Wood, banana peels, old tires, tennis balls and other junk don't replicate human skin, penetration, organs, etc.
2. By bracing your test material so it cannot move backward to absord some of the impact, you will necessarily get more and deeper penetration. A human will necessarily move backward from the impact of a bullet much like he would if you were to punch him in the chest. That backward movement absorbs a significant amount of kinetic energy and therefore reduces penetration. IF you don't understand this concept, try this test: First, shoot your shotgun from a standing position with nothing behind your shooting shoulder. Your shoulder moves backward to absorb the recoil. Next, brace your shoulder against a tree so it cannot move backward. Caution, this could cause serious damage to your body. However, it will illustrate that your test shooting target and penetration are skewed because they cannot move to absorb the recoil. Hence, you'll get deeper penetration.
3. Shooting birds and 60 pound animals cannot begin to replicate a NFL linebacker sized AGGRESSIVE assailant with the purpose of breaking into your house and robbing you, killing you, raping your wife, and molesting your kids. He could be an escaped felon from death row, or on PCP, or carrying a shotgun loaded with slugs. While you're busy waiting for him to get within range and a clear shot, he's firing his stolen .45 through walls and doors trying to kill you once he sees you've produced a shotgun.
I genuinely think, and have backed up my belief with numerous data (as well as the others here that agree) that birdshot has extremely limited application for self defense, in that it would only have any real affect on a BG that is BOTH very close in range AND not wearing any heavy clothing. Conversely, buckshot is much more effective at close and medium range. You may think of scenarios where you are able to engage at close ranges and shoot multiple times, but unfortunately you won't be in control of those scenarios and how they play out. For instance, if the BG is in your home with a handgun and you engage in a gunfight where you are both taking cover. He will win 9 times out of 10 because (1) he's more likely to score a hit due to barrier penetration and (2) his hit with most handgun calibers will likely be immediately more life threatening than any pellet hits you score. And the BG won't care about overpenetration.