Bartholomew Roberts said:Birdshot is not an effective load for 100lb+ mammals at any range. Whether it breaks up or not, it doesn't penetrate deeply enough to be effective. You can look at historical examples, like the James gang raid on Northfield where Clell Miller caught a blast of birdshot in the face at short range from Elias Stacy. The only thing that stopped Stacy from being killed by Miller was Henry Wheeler shooting him with a rifle.
Modern shotguns are no more effective with birdshot. I've talked to people who received a 12ga shell of birdshot from 15' in the upper chest and made their own 911 call. .22LR isn't great; but it is more likely to go where it is aimed (especially with someone less skilled behind the trigger) and it will penetrate better than birdshot.
With due respect for Bartholomew, (because I agree with him 99% of the time) I believe that his anecdotes aren't factual. A full load of #8 shot @ 15' will blow your head off and will certainly more than penetrate the chest cavity, assuming no body armor. One-hundred-year-old "recollections" of the activities of the James Gang don't make for good science. Spend your life in Quail Country, and you'll know a little more about the effects of birdshot at close range. I have attended several murder trials over the years where the weapon was a shotgun loaded with birdshot. Let's use, as an example, a 12 ga. full choke loaded up with 3 1/4, 1 1/4 #8 @ 15'.