I think the bigger risk is internal strife. When half or your citizens hate the other half, that's a recipe for disaster.
It's talking about genocide. If internal strife gets to the point of extreme hatred, then you don't need guns to kill each other. Not saying I agree that would happen in the U.S., but that's what they're talking about. Not Vegas.
To reiterate: See Rwanda. https://en.wikipedia.org/wiki/Rwandan_genocide