I'm of the mind that there is really no difference when it comes to a good self-defense load. I do, however, believe that size could matter and have always used .45 ACP for my self-dense caliber. How nice that it is also the caliber I shoot best. There are specific times when I use 9mm, though, and do not feel at all undergunned, again with a good self-defense load.
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I am the weapon; my gun is a tool. It's regrettable that with some people those descriptors are reversed.