This shouldn't (and won't) become a thread on gun control, but worth noting that quite a few of the mass shootings you see in the U.S. happen in places with significant (or even very strict) gun control. As someone who's not a big gun guy, but knows plenty and has thought and talked about this issue a lot, the one thing I'm sure of is that there is no pat solution.
Well I would point out that this Australian wasn't able to get away with this in Australia so he had to go to a different country that has similar gun laws/ownership as the US.