This is a post I made on a gun discussion board; I have reposted it here. The question was whether birdshot was a good choice for self-defense. My answer has evolved from some years ago, when I said that size T lead shot (.20 caliber; #4 buck is .24) could be ideal. I even said something positive about Federal Cartridge's attempt (since abandoned) to offer even smaller shot in a defense load. But I now conclude that #4 buck is the sensible lower limit of shot size for self-defense use, and it is a case where theory and practice bear one another out.
#4 buck is already on the ragged edge where performance is starting to falter occasionally, in circumstances where the distance is a little bit far or there are heavy clothes or light obstructions involved, and that lines up closely with what you would expect if the military estimate of wounding energy were true, or else close to the mark.