There's more to it than just the weapons though. There are countries in Europe where you can own fully-automatic weapons without any of the dealer licenses and hullaballo required here yet these mass shootings are a lot less common elsewhere. With the exception of the psycho in Norway last year the majority of mass murders have been committed by terrorist groups overseas.
I think a big part of it is that killers here know that their message will be televised. Part of the VA Tech killer's farewell video was shown multiple times on every major network. We found out everything and then some about the Columbine killers, the shooters in MS and Padukah, KY. Every single workplace shooting from the Postal Rampages to multiple office attacks against former co-workers get played up in full-detail. The guy in AZ who shot the senator got his fifteen minutes as well. When you kill here the media gets your thoughts out to the rest of the country and your insanity gets seen in prime time no matter how bass ackwards it is.
If the killers got ignominy instead of fame and didn't get to share their twisted manifesto perhaps they wouldn't see this kind of thing as a way to make a point or get their thoughts heard. The guy let himself be taken peacefully by the cops, and the majority of these dbags kill themselves rather than facing the music against people who can actually defend themselves.
That said, some change to gun laws is necessary. Every interview and tidbit I've caught about this guy shows that the people around him knew he was unstable and even his mother said that she knew he was responsible as soon as he was named a suspect. If someone is that blatant of a headcase then it seems that maybe there should be some kind of competency testing in addition to background checks and that those checks should include inquiries into mental health treatment and hospitalization.
Still doesn't solve everything as the Columbine shooters got someone else to get them weapons and ammo, but it's a start.