This sounds crazy, but just for the fun of argument, let's consider the 22WMR for self defense. This came out of the Keltec PMR-30 announcement. It brings up some points about how we measure performance. Let's start with the basics The good 30 round capacity ultra low recoil ~14" penetration http://www.brassfetcher.com/22WMR.html cheapish to shoot increasing training time The bad limited temporary and permanent cavities I'm going to base this on an average shooter with a < 20% hit rate against say a crack head with a knife. If we are trying to not get stabbed by the guy with a knife, we pretty much have to make a CNS shot. A shot that might be deadly, but takes 20 seconds for the bad guy to pass out and then eventually bleed out, does us no good. The crux of the argument is this: your average shooter misses most of the time under stress. If you can improve accuracy even slightly it significantly increases your likelihood of incapacitation. If shooting a 22WMR increases accuracy more than it decreases instant incapacitation, it's a better choice. So let's say we have 2-4 seconds to stop the bad guy. Our sub 20% hit rate with 9mm or .45 puts us at 1-2 hits. The likelihood of instant incapacitation is really low. Given that we have lots of penetration out of the WMR would doubling our accuracy or speed make up for lack of permanent cavity? I believe that while a 22 is going to reduce the amount of deadly lung shots for example, accurate CNS shots may not be reduced because of increased accuracy and number of shots placed on target. This brings up some important points. It seems that most of our research is centered around terminal effectiveness. But terminal effectiveness is not our goal. Terminal effectiveness per firefight or terminal effectiveness as a function of time is our goal. If we reduce effectiveness by 20% but we increase shots landed in the same amount of time by 30% we are ahead. Another problem is the dichotomy between theoretical performance and anecdotal performance. We constantly here "I put two X calibers in the guy's chest and he didn't go down. A 45 would have done it." But we never hear "I dumped an entire magazine of 45s into the sky, and he didn't go down" because no one brags about how bad a shooter they are. It's much more interesting to hear about the .22 that bounces of the guys forehead so it get's discussed way more than the fact that 22s kill more people (though usually from ND). We can't base our strategy on anecdotal performance that doesn't measure the uninteresting stories. What do you guys think? Obviously, larger caliber makes sense for people who are highly skilled, and we should all be on our way there, but for an average shooter, could the reduced recoil/increased accuracy of a .22WMR be more effective?