It sounds like you want to keep them as friends, which I see as understandable. I always turn it around on them. When you argue with an anti-gunner you are basically accepting their argument and are on the defensive. I simply refuse to take the defensive and go on offense.
Once when my boy was around kindergarten age my wife left me to make arrangements for a play date with one of his school friends. The mom called me and we chatted for a while making the arrangements then she just straight up asked me if we had guns in the house. So I said yes, explained that they are all in a safe, blah, blah, blah. She wasn't having it and started lecturing me about how I would be putting her kid in danger if he came over to play. She then informed me (in her most condescending liberal way) that it would be ok for my son to go over to her house. So I flipped things around on her and asked her if they had any guns in their house.
Me: Do you have guns in your house? Even for protection?
Her: No.
Me: So if something happens over there you have no way to protect my boy? You know he can't be replaced right? I can make more children, but I can't get him back. He's my pride and joy, blah, blah, blah. (I laid it on thick)
Her: Silence.
Me: You still there?
Her: Yeah, maybe we'll have to do this some other time.
Me: Sure thing. Have a good day.
Haven't seen them since, don't miss them.