I tried to find the tests for birdshot vs. buck at the “Box of Truth” website but couldn’t. With some more searching you probably could if you are interested. Lots of stuff in there.
Thanks for the heads up. I had forgotten about that website.
After reading several of his test reports, it seems he’s concluding that, although birdshot won’t generally go through more than one layer of wallboard, it doesn’t have the power to penetrate vital organs, so it doesn’t stop the bad guy.
Therefore, he recommends #00 and careful targeting.