Shootings by bad guys tend to last until the first good guy (cop, or not) with a gun arrives.
More guns in responsible private hands in the U.S. results in fewer mass shootings with less victims.
The bad guys will get (or make) guns if they want to. Even in prison. The technology is well understood by anyone who wants to learn it.
So yeah, we'd like to limit mass (and other) shootings by increasing the number of people capable of stopping them. As it turns out, no increase in stricter gun laws is known to have resulted in a lower crime rate.
What's amazing is that even though the Democratic process by which laws are passed in the U.S. is clearly not in favor of stricter gun laws, somehow a supposedly Democratic President refuses the "will of the people" and wants to do anything he can to unilaterally and potentially unconstitutionally (with the mental health stuff) circumvent it.
If the issue is new laws need to be passed that clarify and fund enabling people who are an actual threat to others to have due process of law and have their rights taken away (including getting locked up in an institution, a process the folks with a D after their name destroyed about 30 years ago or so), then yeah, let's have that debate in Congress where it (and other new laws) are supposed to begin.