been thinking about this for a "trunk" or "truck" gun, don't have a trunk or a truck at the moment so mine would be a "hatchback" gun, but indulge me.
H&R "Survivor" rifle in either .308 or .223.
Single shot, break open action. Synthetic stock with storage space in the butt stock as well as the forestock to fit a decent amount of survival gear in as needed. ie, small knife, fire starting tools, etc.
Sling with ammo carrying capability or a butt stock carrier to have 20-30 rounds on hand with enough other tools stored in and on the gun itself to get you home if the worst occurs while on the trail.
Very reasonably priced, under $300 from any dealer for either version. Maybe not the absolutely most needed piece of gear in the world, but plenty of cool factor points. From what I've read they're plenty accurate for meat hunting. Also available in a shotgun/rifle "Handi Rifle" combo setup but the hollow stock won't fit according to the folks at H&R.
There's a great thread about modifying one as described above on the Zombie Squad forums.
You Reading This: Stop
Don't just stay tangled up in your life.
Out there in some river or cave where you
could have been, some absolute, lonely
dawn may arrive and begin the story
that means what everything is about...
William Stafford 1914-1993