There are possibly as many as 100 threads about this. THe accunulated wisdom seems to be that preparing only one stock answer will leave you in the lurch more times than it will save your bacon.
Guaging (guessing) the motivation behind the question can lead to positive conversations about not only why that handgun is hanging off your hip, but things like personal responsibility, ways to control crime that do not start with tax increases, or even why pick utilitarian kydex when leather is so much more esthetically satisfying.
Like any good trainer, you should know enough about your subject to talk, without notes, about it for 5 minutes. not that you necessarily want to do that - most of the time it is better to get the other person to talk to you. Your job is to direct the conversation - away from mindless anti-gun screed and towards actually exploring the question of "Why?" and how they feel about it as well as why they continue to support laws that do nothing to prevent crimes/shootings. (Remember - get them talking about those things. THey do not want you lecturing them.)