For carry, often the platform is MUCH more important than the caliber. You can only get a .45 so small...even an XDs isn't all that small.
For example, for a true pocket pistol, even 9mm (I'm thinking PM9) is a hair too big. So in that category .380 shines.
For IWB, strong side, your options REALLY expand. If one is willing to go double stack you can really get anything you desire. 9mm is fine.
.45acp, .40 S&W, 9mm, .38 Spl, .380 acp, 22lr
12 gauge, .223/5.56x45mm, 30-30 Winchester
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