It's worth mentioning the example of Australia, as well:
In 1996, Australia changed its gun laws following a particularly bad mass shooting: banning assault rifles, severely restricting other types of fire-arms, limiting magazines to five rounds (three for pump-action shotguns, auto-loaders were banned outright). Researchers compared the rate of mass shootings before and after - using fixed criteria, not just what "felt" like a mass-shooting - and found that the rate dropped from one every 18 months before the change, to just one event in the 16 years since the change.
There were no other noticeable changes that might be responsible for the reduction; no reduction in poverty, or improvements in mental health treatment. And judging by other crime rates, there wasn't a significant change in culture or economics or policing. Hell, even the number of firearms in society recovered within a few years. And the only things that did change in Australia, it shared with the US. There was an increase in antidepressant use in Australia, but so too in the US. Video-games violence became more photorealistic, and so too in the US.
It's such a perfect experiment: Similar culture. Only one major change, gun laws. And one clear result, the virtual elimination of mass shootings.
If you want to reduce the rate of mass shootings in the US by an order of magnitude, Australia can tell you how to do it.
But your country won't like the taste of the medicine.
Indeed, in USA, many DE FACTO accept mass shootings as the one that just happened (as long as they are not the victims, of course) if that's the price to pay for them to keep having assault rifles, high-capacity magazines, etc.