I believe while interesting, the issue of which gun to carry plays a much smaller role than just having a gun to pull when needed. If you have to pull a .380 on an aggressor, do you really believe they'll look at it and think, "I can take 3 or 4 hits from that pop-gun, no problem", and will continue the attack? Or will they think, "Schite! a GUN!! Runnnnn!!!!" How many people do you believe would attack you if you pumped 10 rounds of .22LR right into their face?
Yep, most of the time that would work good enough. But sometimes it wouldn't. That's not to say that anything less than 9mm or .38 Special is an ill advised choice, though, since it's all a matter of finding a balance, and where everyone finds it is different. For me, it's 9mm/.38 Special as a floor. For others it could be .380, .32, .25, or even .22 rimfire. For some, it has to start with a four.
