For people who are willing to engage, socratic dialog can be effective. For example, they want to ban guns but what is it they are really after? Banning guns is just a means to an end. They'll say they want to stop gun violence. What about violence not caused by guns? Do they want to minimize that as well? Is all violence bad? What about justified violence, such as in self defense? Do people have a right to self-defense? Isn't taking away the means of self defense the same as taking away the right to self defense? Is banning guns the best way to reduce unjustified violence? Are there alternatives? This helps them better understand their position and exposes the internal contradictions. I don't think you'll find people willing to engage in dialog at a pro-gun rally though. They'll just scream in your face about dead kids.