Some time ago, I stumbled upon this instructive video on shotgun ammunition.
When you come to speak about shotguns, the question regarding the appropriate (home) defense ammunition almost automatically arises. The most important factors to consider are:
1) Expected distance of engagement
2) Stopping power of the rounds
3) Penetration capacity
I recommend reading this analysis:
With the right load, a shotgun can be very effective in quickly stopping the deadly violence being perpetrated by a criminal who’s invaded your home.
If you’re worried that a missed shot might penetrate through a wall and harm others, load your shotgun so that the first one or two cartridges to be fired is number 6 or smaller birdshot, followed by standard lead #1 buckshot (12 gauge) or #3 buckshot (20 gauge). If your first shot misses, the birdshot is less likely to endanger innocent lives outside the room. If your first shot fails to stop the attacker, you can immediately follow-up with more potent ammunition.
With birdshot you are wise to keep in mind that your gunfire has the potential to NOT PRODUCE an effective wound. Do not expect birdshot to have any decisive effect.
Number 1 buckshot has the potential to produce more effective wound trauma than either #00 or #000 buck, without the accompanying risk of over-penetration. The IWBA [International Wound Ballistics Association – ed.] believes, with very good reason, that number 1 buckshot is the shotshell load of choice for quickly stopping deadly criminal violence.
Also, this page is worth reading.