Actually you did pretty well except here:
PO: Did you know its illegal to have a gun in your car?
ME: Really? How do people go to shooting ranges?
PO: So, are you getting back from the shooting range?
He was baiting you and you took the bait rather than remain silent, then you gave him a reason to go fishing for probable cause. He could have easily searched and then said he thought you said you were at the range and were transporting weapons, even though that's not really what you said.
Every time you open your mouth you are risking the officer hearing or interpreting something you say as either a waiver of your rights, or as something incriminating. That is why it's best to remain silent if you are asked any questions, ESPECIALLY if you are already in handcuffs. Overall I think you did very well though, you did avoid a search, which could have caused you much more problems depending on what the officer thought about what was in your car.
By the way, PC 148 is resisting or obstructing, and exercising your right to remain silent cannot be prosecuted under that code. If you actually lied and said there were no weapons when there were weapons, then MAYBE he might have something.