The most important reason to not use a 22 for self defense is reliability.
Rimfire cartridges are less reliable
I make it a point to fire at least 500 rounds of .22 a month from either a scoped 10/22 or a small 9 shot revolver I have. With the rifle I can put almost every shot inside of a half dollar at 75 yards.
I can put every shot from the pistol inside of a small paper plate at 7 yards in less than five seconds. I don’t use either for my EDC obviously, but if push came to shove I know I could stop Mr. Bad Guy with that little revolver.
The .22 in your hand will beat the 9mm you left at home every single time.
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