The AR-15. I don't want to derail this thread because it has been a good discussion thus far, but need to point out that semi-automatic rifles have been around since the 60's, I mean, the Marlin Model 60 has been around since, literally, 1960. You can dress these guns up all you want, wood stock so it looks like a "hunting rifle", or carbon stock with all the attachments to make the gun look like an "assault rifle", but in the end, functionally they are the same. So when people talk about banning semi-automatic "assault rifles" they are also talking about banning a .22 plinking rifle.
I point this out because these discussions always seem to go back to the guns, specifically the scary AR-15. The reality is that semi-automatic rifles have been with us for a long time but school shootings are a relatively recent thing. I think focusing on guns is focusing on the wrong thing. We need to address the core issue and that is messed up kids who are so effed up they resort to the mass killing of people.