"Do you think you're a tough guy beacuse you have a gun"
"No. Do you because you don't?"
This statement means nothing, but I have used it 5-6 times, and for some reason, people just shrug and walk away. Whatever works, I guess.
I was also asked once: "Why do YOU have a gun?" "Target practice". Dead silence. They had no argument for it.
Also, when people use the "if we can save one child's life" line, I'm partial to "I agree, let's ban unsafe things to save children's lives, that way people will have no responsibility. Let's get rid of swimming pools, bathtubs, and cars first. Then stop, as soon as they open their mouths, add plastic bags, power cords, knives, ladders, stairs and screwdrivers. That will usually end the debate.
(Slightly off topic, but still funny, I thought: one time while explaining to my wife how a semi-auto works, then how a revolver works, she was concerned about the revolver not having a safety. She asked "Doesn't that make it unsafe?"
"Depends on which end of the gun you are standing.")