Truck gun, now that's something I can talk about! I say that because I've done alot of varmint/predator hunting from the truck here in Wyoming. Alot of it at night with spotlights. We need something that is very flat shooting because alot of times we dont have a range to the critter, we just know it's within range and about to be out of sight or headed for the next ridge/ravine. Most of the time there is no time to be figuring distance and where we want to hold. That time should be spent getting into the most stable shooting position we can.
For jackrabbits, prarie dogs, skunks, stray cats, wild dogs, coyotes, fox, bobcat, raccoon, rock chucks, badgers, porcupines ect. I used a 22-250 and a 220 swift for a long time, both medium weight 26" barrels. 26" barrel is really too long for getting out the window quickly or jumping in and out of the truck all night to shoot over the hood or the bed.
So, next I tried 223 AR-15's. Short enough barrel, but brass was always flying everywhere. Hitting the windshield, landing in my coffee, hitting the guy in the passenger seat, falling down in the defrost vents and between the seats, and just flat out loosing too much brass if I was outside the truck...especially in the dark. Not to mention, they're not as easy to unload or make safe before jumping back into the truck to look for the next jackrabbit. It's real easy and quick to just cam open the bolt with a bolt action, but an AR needs to be unloaded and locked back, or remove the magazine before letting the bolt close again........more steps to get to the same place is all.
Latest gun I think will take the cake. It's a 20" barrel bolt action Remington, chambered in 204 Ruger. The gun will hold 7 rounds, it's light and short, and very flat shooting out to most "truck gun" distances.
Biggest thing, regardless of the caliber you pick; is get something light, short and manuverable. Mount the scope so you can very quickly find full field of view. Last thing we want when spotlighting/truck shooting predators is to have to fiddle around and find the scope with our eye. It should be natural and instictive. Most LR dedicated scopes with high mounts are not very good for this type of thing. AR's have pretty high mounts, and found them harder to get into position very quickly.
Good luck, lots of fun. Lots and Lots of fun!