Originally Posted by mobuck
The best time for the physical defense is BEFORE the first shot. Before the shooter works up to actually shooting there's some indecision and possibly some confusion that can allow a successful physical attack. A physical attack from multiple points will often confuse an inexerienced shooter allowing contact to be made.
Sure, that is the ideal time to stop them, but that option is rarely apparent in social situations. Unless you catch them reaching for a gun, (or wallet, or handkerchief, or keys, or smokes, or nose picker), you aren't going to stop them preemptively. And of course, planning for the worst situation may be a fantasy as much as expecting the best situation, but as long as you're sticking to a general plan you are better off.
Personally, I don't think enough people practice to respond to the sound of a gun shot and people yelling and panicking.