Bogie, you've raised a good question.
Glenn, your answers are superb and well thought. But your best points were buried in the verbiage.
People need training in empirical methods to rationally examine positions on the basis of the merits of the issue. Many folks don't - they are socialized to the emotional, playing on fears and the like.
They are taught to be self-centered and anti-intellectual.
One difficulty is that once a person has invested any time or intellect in supporting one side or another, their emotional interests usually outweigh or cloud the ability to rationally examine the facts.
But let's get into some specifics. You aren't going to change the mind of a hard-core anti-gun person, not even with the most impartial of facts. So ignore that part of the equation for a moment. The focus has to be on the "ends justify the means
Someone who defaces property because it holds a pro-gun message or pro-something
message have a form of self-delusion and/or mental illness. This would include any of us who might deface property because of a "No Guns!" sign.
Those who push such actions as acceptable need to be interrogated as to when such actions are NOT acceptable. If it's okay to deface a car with a McCain/Palin sticker, is it also acceptable to trash a Prius with the Obama sticker? Why not? The response is likely to be informative about the mindset of the person.
The follow-on question is, of course, Supposing your justification is true, is there ANY law you won't violate to support your cause?
Once they admit there are certain laws they'll obey, then the question becomes why it's permissible to break some laws, not others. And consequently, if you can lead them to justify violating some really serious law like arson or murder then step back and paraphrase it like "so you think it's okay to burn down people's property or kill them if they don't follow your view?"
Of course, this presumes you can actually carry on a dialogue with them. Usually attempts at a serious dialogue are met with ridicule, hyperbolic statements (you'd let 3rd graders take machineguns to school
!) and frequent forays into off-topic subjects like racism, social inequities, class warfare, ad nauseum.
Discussing flawed research is usually beyond the scope of competence for most of the anti-gun crowd. When I pointed out that Kellerman's 43-times study exempted lawful uses it was dismissed as irrelevant. But when I point out that his study is tantamout to studying only the survivability of plane crashes while ignoring the vast majority of safe flights, it gets a little more thought (albeit not very much in some cases).
If you listen carefully to left-wing speakers on media shows, they try to monopolize as much time as possible in rapid fire to prevent questioning of a point or continue talking regardless of someone calling them on something. When they do the same thing in a personal discussion or small group discussion, it's the verbal equivilent of putting their fingers in their ears and singing.