RE: shotgun or rifle behind the truck seat.
In Kansas, it is unlawful to have a chambered round in a firearm in a motor vehicle.A full magazine isn't considered loaded as long as the chamber is empty. As far as concealed behind the seat, it would have to be in a case to be considered legal. Without a case behind the seat or under the seat is concealed. I usually have my guns in my front seat in the middle while driving to hunt without a case.