Just my 2 cents worth on this but I have been stopped a couple of times with rifles in the car and always found that open/friendly/chatty responses got the best results. Because I know I'm legalit's easy. If I was a cop and someone gave me an ACLU minimal answer it would immediately raise my hackles.
Q: Have you got a firearm in the car?
A: Yep I'm on my way to the range at Chabot (or wherever) to try out my new toy. I'm super excited. Want to check it out? You guys must get to practice during work hours, that would be awesome etc etc