My overall favorite would be the .22lr.
*Can be used in either semi-automatic pistols or revolvers.
* Very economical to shoot, you get more bangs for the buck stretching the
budget to allowing for more practice and plinking.
* Takes up little space & weight so a goodly amount can be carried with a small pistol or revolver (kit gun) in a backpack or with your fishing tackle.
* Good for taking small game & pests.

No, I would not use it as a defensive round unless forced to by circumstances, but the question was not what is your favorite carry or self defense round.

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Life Member B.A.S.S., NAFC