Again, people advocating this are acting like there is one solution to mass shootings in schools or public places - more firearms in the hands of more people.
If only the teachers were armed. . . .
If only more citizens were armed. . . .
If we did away with gun-free zones. . . .
Gun violence for any reason is a more complex problem. Contrary to what some assert, there is no clear statistical evidence that more guns in the hands of civilians can stop gun violence or lower crime. It certainly hasn't in the US, where citizens have more guns and more types of guns per capita than in any other advanced nation. The countries that rank above the US in homicides are developing or third world counties where guns are plentiful. Canada and all of the advanced nations of Europe that have more regulations on guns rank lower in homicides.
I am not against gun ownership, but the US Government asserted long ago that the right to own firearms can be regulated, particularly with the advent of modern automatic weapons, which were first regulated under the National Firearms Act of 1934. All the rights in the Bill of Rights can be regualted in order to protect US citizens and the welfare and security of the nation, just as freedom of religion was regualted in the case of polygamy, the freedom of assembly is regulated in some cases by requiring permits, and freedom of speech is regualted when speech is used to incite others to violence.