when I actually owned a truck, my truck gun was a Mosin-Nagant M44 secured by one of those rubber coated hooks for hanging bicycles. I bent the hook so it would hold the rifle, but still come loose if I yanked it from the drivers seat. I liked the m44 because it was solid, reliable and inexpensive. If it got messed up or stolen, I'm only out $80 or so. It gave me plenty of power for anything I'd ever need and I could keep extra ammo in the door pocket or center console.
when I sold the truck, my "truck" gun became my FN Forty-Nine in .40S&W.
It handles much better when I'm in my VW GTI.
-edit- my truck was an 84 Ford Bronco and the rifle slept under the seat.