The suspect in the Paris attack

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14. If you're not clear and immediate danger, you don't shoot at all.

If you are, you fire at the center of mass to both minimize the chance of a stay round striking an innocent or a fellow LEO and to maximize the quickest end to the danger.

Shots to an appendage may not provide sufficient trauma to induce shock quickly enough to disable. A shot aimed at a thigh could go a bit high or low and graze a butt or calf. People in a lot of pain tend to get pissed off and take that shooting them thing personally.

There may be a small number of extraordinarily accomplished shooters capable of that but hitting a man sized target during a stressful moment from say 12 - 25 feet is not as simple as it seems. It's even more difficult if that target is also doing stuff like moving and shooting back.