http://www.ronpaulforums.com/showthread.php?398679-62-Mass-S... is I think my final point here; it's a sympathetic gun rights advocate who went to the trouble of tracking and linking local ordinances where possible. He's only proven the case in about half the incidents. I know John Lott claims he researched all of them. If you happen to know where his supporting documentation is I'd be sincerely interested.
I might be looking at a full on amendment here, or there may have been more statistical jiggery pokery that somehow excludes findings incongruent with the expected results. Either way, I'll do more research and get back. Thanks for the correction -- while the link you posted isn't necessarily conclusive, it did spur me to freshen my data sets at the very least.
Calling mass shooters stupid is, I think, a naive assertion. They're obviously flawed, but not necessarily in a way that makes them less effective at planning or committing violence.
And while 'c' is potentially valid, I think that what you'd find is that, as most citizens tend to fall into the 'law abiding' category, where there are signs posted disallowing guns, you'd find that most citizens won't carry them. So even if the 'gun free zone' is not found to be 100% devoid of guns, you are almost certain to find it less armed than if signs weren't posted.