the one-shot, one-kill theory is movie fiction.
In practice, a bullet will stop someone cold only if you hit a vital organ like the brain or heart.
Shooting someone somewhere else will only cause loss of blood and a human being has to lose 20% of its blood before it loses consciousness and is no longer a threat. Obviously shooting someone multiple times will accelerate the progress. That is the reason why you are taught to keep firing until the target is down.
As noted above, there have been cases in Iraq and Afghanistan of individuals suffering multiple bullet wounds without any visible effect. It has been theorized that the ammo used by US forces (5.56x45mm, basically a .22) was going straight through causing minimal damage, meaning the person could continue fighting until loss of blood caused him to lose consciousness, which could take many minutes.
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