Just thinkin' out loud here now, but what I'm curious to know is whether owning a gun actually makes a person safer. I suppose I'm also interested in how many times gun owners felt they needed their gun (even if only to show it, and not fire it), but that's not the same thing. In fact, the difference between those two numbers could be revealing, in itself.
I'm thinking of a hypothetical study of gun owners and non-gun owners who live together in the same cities or counties, so that they would be exposed to the same threat-level, whatever it may be. Are the non-gun owners more likely to be victims of crime? If they're not, it suggests they're taking other steps. Of course, it's also possible that non-gun owners are more constrained somehow, in their efforts to mitigate risk without a gun, in ways that gun-owners are not. At the same time, we'd need to find out what crimes are created or exacerbated by the presence of a gun, in order to weed those out as legitimate DGUs.
At least superficially, without giving it a lot of thought, I'm not sure what the simple number of DGUs really tells us. Among other things, I simply don't believe that most people are good judges of when a gun needs to be used and when it doesn't. My basic problem with "stand your ground" laws is that they empower every Tom, Dick & Harry to decide, in the heat of the moment, that someone else needs to die. On the face of it, that just sounds silly, like letting people decide for themselves whether they're okay to drive after a few drinks. I think the expectation has to be that it's a bad idea, and that testing the hypothesis in real time, using the general population as unwitting test subjects, is an unethical experiment.