Argue against the idea that the weapon used in a mass shooting is the main cause of the tragedy, and use a hypothetical situation to demonstrate the flaws of this idea.
Your child is in a crowd at a concert. You must choose the weapon the lunatic on the 32nd floor has. Are you going to choose a semi-auto rifle? Didn't think so. I'm going to choose: alcohol, knife, household cleaners, automobile, .... See how silly that premise is.