For me a truck gun is a short barreled rifle that is easy to maneuver and quick to get out the window for shooting hogs and coyotes but also occasionaly a snake, maybe a deer, etc. I prefer something inexpensive to shoot as well. A short light bolt gun with a low powered scope is ideal. Like a .223 with a 2-7x or 3-9x scope. Some of the little, light 7.62x39 bolt guns have my interest at the moment for a truck gun. Cheap to shoot and enough accuracy and power to thump a coyote or pig when riding around.

I also carry a handgun in my truck but I don't really refer to that as a truck gun. I keep a mid-sized 9mm in there usually.