This argument is an absurdity. It is also irrelevant. We already have visible deterrents to free speech that have decimated the marketplace of ideas in college classrooms across America. I am, of course, referring to speech codes. And some universities, like my own, are so committed to these codes that they are actually visibly displayed on classroom walls.
Walk into any classroom at UNC-Wilmington (where I teach) and you will see a framed "Seahawk Respect Compact" displayed prominently on the wall. It warns students to refrain from using any words that might be deemed "disrespectful" or "uncivil." The problem is that, as Justice Oliver Wendell Holmes once stated, "every idea is an incitement." In other words, people can respond to any idea they don't like by complaining that it is "disrespectful" or "uncivil" - or "offensive" or "unsettling" and so on and so on. The problem is exacerbated by intolerant leftist professors who actually think these speech codes trump the constitutional rights of college students.
Put simply, our constitution protects offensive speech. It doesn't protect offended individuals.
Speech codes are a far greater threat to free speech than handguns. The reason for that is simple. No one thinks he'll be shot by a gun toting conservative just for expressing a liberal idea. But many people fear being prosecuted by a speech code toting liberal just for expressing a conservative idea.
The reality of this is that replacing "respect compacts" and other speech codes with guns would increase respect and civility in more ways than one. Both censorship and crime would decline precipitously.