I guess my definition of a truck gun is different those most of what's being posted here. I wouldn't call the ones posted truck guns but Personal Defense and "Look at Me" Guns. We don't consider our truck guns to be for defense but for takin' game. This is more in line in what is traditionally held to be a truck gun in our patch of the boonies...
Truck guns, sometimes thought of as trunk guns have been traditionally though of as:
1) something you wouldn't mind losing
2) something that you wouldn't mind getting damaged / beat-up
Though I would add....
3) Something ready to take whatever game is in season when you stumble across it.
4) Something reliable and capable of taking abuse
5) Something not for a full day of hunting, but available when you find the need for it
6) Something that if it gets stolen isn't going to cost and arm and a leg to replace or tear you up with the loss of memories it gave you.
Mine is an old 870, bought real cheap years ago and it now rides around in the truck. It is always ready to dispatch a tasty morsel that I might come across. In it's case are a few different choke tubes if I want to change up the pattern. There's a couple of boxes of shells in the truck from game loads, turkey loads, and some slugs for larger critters. Come fall a couple of boxes of steel shot end up in the truck when the ducks and geese are flying.
This old gun has taken everything from squirrels, rabbits, game bird, turkeys to geese, ducks and swans. Never taken a deer 'cause in hunting season the "Deer Truck Gun" gets the call for that. It's well used, shot a ton of critters, gets limited care and is always dependable when the time comes for it to shine. It ain't black, doesn't have any fancy sights, got an old faded camo paint stock but it's a hell of a good Truck Gun.