I went into a convenience store not long ago and was carrying open. When I approached the counter, the cashier saw my gun and holster and made the same comment mentioned earlier...."remind me to never piss you off!" To that I simply replied, "oh you can piss me off all you want to, that's no big deal.....just don't endanger my life". I said this in a very casual non-threatening way, and kept a friendly tone of voice and smiled. I did NOT want to go on the defensive and go off on some big speel about gun rights/self defense. He was very receptive to my reply, and just chuckled and said "alright, well that's cool" He smiled and told me to have a good day as I was leaving and I told him the same.....turned out very friendly and positive.
Some people however, are just as far to the left as I am to the right, and with them it's best to just agree to disagree and leave it at that. No need to stir up a heated debate as this usually leads to nothing positive. In those cases, I maintain a positive and friendly demeanor and simply move on.