I'm not sure, but as there are over 50,000 mass shootings in the US most years, I'm gussing a lot of them have legally obtained firearms.I'm not one of the pro gun guys (nor am I anti-gun), but I wonder how many of the mass shooters used guns that were legally obtained?
The answers to why these things happen and how they can be prevented are never a single answer and solution. Too many people think they are.
And I'm not saying you're thinking too narrowly, as you claimed (rightfully so) that it's a place to start.
I really hope this doesn't spark the absurdity that's all too common.
http://www.gunviolencearchive.org/past-tolls