A .22 can do the job, but other choices are better. Shot Placement is the key, if you can turn off the motor it's over, unfortunately many who ended up eventually killing an attacker were themselves killed by an aggressor due the SLOW BLEED-OUT.

You either hit the SWITCH that kills the central nervous system, or wait for the Bleed-Out to occur. Bigger wound channel equals a faster bleed-out. Those two situations end the encounter permanently.

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NRA Pistol/Rifle Inst. RSO