I don't think Mr Donston ever got his answer, I'll take a stab at it. Police training is focused on stopping attackers, not killing them. Not every deadly force situation has a deadly force solution. A police officer will know that every person with a gun is not necessarily an attacker and will not shoot every armed person he encounters.
A few thoughts on charging: have you ever tried to engage and shoot a rapidly advancing target? A charging target that presents a threat is remarkably hard to hit. Handgun bullet wounds are often survivable with the right mindset and immediate medical care. Another thought, wouldn't it be nice if two or three folks got the idea to charge a shooter at the same time? It happens, and sometimes it works quite well.
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