While this discussion pertains to the incident in Colorado, I feel that I must mention the incident that took place in Norway.
https://en.wikipedia.org/wiki/Anders_Behring_Breivik

ALL of the regulations in place will not stop a determined individual.

I've also lost count of the rising death toll in the "drug wars" along our Southern border. In case you're unaware, Mexico has very restrictive firearms laws, yet there are a LOT of full auto weapons in play down there.