Put the gun(s) in a locked case, in the trunk. In some states, police are told to ignore the FOPA; they will arrest you and someone else will decide whether to prosecute you or not, after they confiscate your guns and keep you and your family in jail for a day or two just for the fun of it. If you are travelling through a state where the law is tough on gun owners, the main thing is not to do something stupid that would invite police to stop you or search the car if they do. That means:
No "OFF THE PIGS" bumper stickers
No "COLD DEAD HANDS" bumper stickers
No zig-zagging from lane to lane
No running toll booths
No excessive speeding (in some places, holding to the exact speed limit means you are either nuts or drunk)
No flipping the bird to a cop
No objects in the car that would lead a cop to think you have a gun in the car; that means pistol magazines, reading magazines like Guns and Ammo, speed loaders, ammo boxes, etc.
No other illegal objects in the car that would be visible to a cop. Stash the bong or the smoky spoons in the trunk.
No blowing MJ smoke in the cops face while he is checking your drivers license.
Jim