Stephen Williamson suggests we might want to tax guns because of the negative externalities involved. A commenter of mine agrees.
But this is all pretty presumptuous about the effect of guns on third parties. It's possible we should subsidize guns too. I'm not a gun owner, but I do know and trust my neighbors. They're pretty good guys, and they've already looked out for Kate and me on minor things. I have no idea if they are gun owners, but if I found out they were I'd feel even safer.
This is pretty straightforward stuff. Of course guns can cause harm to third parties, but they can also benefit third parties.
What wins out? Should we be subsidizing gun purchases? If we're going to invoke Pigou on this one we ought to consider this possibility too.
I honestly don't know what wins out. It's not my sense that the evidence is clear either way. My guess is that there is a fairly large gross negative externality as well as a fairly large gross positive externality, and I'm not sure where the net effect comes down.