You may be right, ymb, but it's hard to imagine that more stringent gun laws would make this sort of thing more likely. It flies in the face of almost all empirical evidence, let alone common sense, to suggest that making it harder to obtain such large quantities of guns wouldn't have an impact. I am sure that it wouldn't remove gun crime entirely, but not being able to solve a problem completely is no excuse for not trying to solve it partially.
The problem the US faces is that, firstly, the guns are often already out there for many people anyway, so you would have to not only stop selling guns so freely but also try and encourage or force hand-ins of the guns people already have. I think that was done in Australia, and shows that the principle works, but it would be a much larger exercise in the US.
But the second problem is that there's not enough will to take that step, either among the politicians or among many US citizens. I think, again, the reddit post I linked to above summarises the point rather nicely.