[microcolonel]

> Also, how come the incidence of these types of shootings are so much lower in other parts of the world if there is no policy that will help prevent them?

Well, the short answer is that deaths from incidents like this are actually more common, considerably more common, in some other parts of the developed world[0]. The majority of U.S. mass shootings result in no deaths, the overwhelming majority result in one or fewer. Only about 70 of the 474 or so mass shootings in 2017 were also classifiable as "mass murders" (single incidents resulting in four or more unjustified killings).

[0]: http://www.politifact.com/truth-o-meter/statements/2015/jun/...