True, but adopting a variation of the Japanese model for permitting gun ownership may have helped (http://www.chicagotribune.com/news/opinion/commentary/ct-perspec-japan-guns-buying-process-1009-story.html).   An adaptation of their method to the US could be to oblige that anyone desiring to purchase a gun exceeding a lethality (e.g. rate of fire and caliber) typical of a hunting rifle would be required to go through training, interviews, and investigation similar to the Japanese model.   Multiple gun purchases within a short period of time should be another flag.

We'll never eliminate the possibility for another mass shooting but we can take common sense steps to reduce the risk that do not conflict with the 2nd Amendment.