If traveling:
1. Carry your unloaded guns and ammo in a locked container, preferably not in the passenger compartment. It is legal to do so, but visible weapons cases WILL result in increased "interest" and questioning if you are stopped. That which is not seen does not exist.
2. If stopped for a traffic violation, provide your license, registration and insurance. AND KEEP YOUR MOUTH SHUT.
3. If a cop asks you if you have any weapons in the car, do not lie, BUT DO NOT ANSWER. He is fishing. He can ask, but he cannot require you to answer. If he insists, tell him you decline to answer. The good old fifth amendment protects your right to do so.
4. There are only two ways a cop can search your car, your consent or a warrant. If he says, "Pop the trunk please," he is asking for your consent. Do not give it. You will get the classic, "if you've got nothing to hide...," "Why are you not cooperating," etc. He is bullying you for you to give consent. NEVER NEVER GIVE CONSENT TO A SEARCH. If you consent, your right to complain about the search has been waived. If he says he is going to get a warrant, HE IS PROBABLY BLUFFING, again bullying you to give consent. At that point, ask him if you are under arrest, or are you free to go. If he does not have probable cause to arrest you, he does not have probable cause to search either. And a recent (I believe Ohio) case stated that a "reasonable" detention for the purpose of investigation cannot exceed fifteen minutes.