Whenever some anti-gun advocate says something to the effect that people carrying their personal defensive tools are trying to live out a fantasy and they would never be able to use them in a real situation anyway we should invite them to play a game of laser tag to confirm their claims in regards to a mass shooting situation.
Arm about 10% of the potential victims and then let the anti-gun bad guy see how far they can make it as they try to play “postman” in the mall, schoolroom, or crowded health club. Repeat without the potential victims being armed. Compare the results and then discuss which scenario played out better for the potential victims.
If they refuse to play then tell them they are living a fantasy and are unwilling to gather and process factual data.