I have found it best to stay away from ideological or philosophical arguments when debating the indoctrinated. I put things in simple terms. For example, it's not guns that are bad, its the bad guys who use them illegally...otherwise we would not be calling cops with guns to respond to crime. I also show the negative effects of bans by asking what side of a state/county line you would rather cross in order to rob someone...the side with a concealed carry law or the one with an outright ban?

In the end, it boils down to mistrust. Anti-gunners trust cops to have guns, but not law-abiding citizens, because they don't think of us as law abiding citizens, but as folks who are one lost job or argument away from becoming a killer. The more we interact with the public responsibly, the more we can remove this bias.