Hmm...
Price point or capacity needs to give some. I wouldn't give on capacity, but would on price point.
AK folder or AK & put a side folding stock on it. Sturdy, capacity, known hardiness etc. can't stay loaded though.
AK pistol, but I'm not that familiar with them, could stay loaded though.
(--oops RE read your criteria, .30 or better, so disregard my AR pistol suggestion bellow)
AR pistol idea is a really good one. Do you allready happen to have an AR pistol lower? Or a lower not "registered" as a rifle, so you could use as a pistol lower?
--We have an Adams Arms 5.56 7.5" tactical Evo pistol complete upper I'd been thinking on selling for $475. Swapped it out for a 300AAC.
So I'm thinking a pistol of some type might be the way to go. Lighter, much easier to conceal (and be legal), and ready while concealed. More difficult to reach out to 200 yards with any accuracy though.
The other thing to consider is what types of threats are you considering defending against, how would you store the weapon & ammo, and how quick for you to be able to gain access to it AND be at the ready.
Suppose if you have a decent side arm, with an mag on you & a few extra loaded magazines in your car, to give you that little extra time to get to the trunk gun...then any trunk gun. Then you can use whatever platform you are most familiar with as the trunk gun. Just take one from your safe & put it in there, it hasn't disappeared, you still have it & can take it to the range anytime you want.
So in that circumstance I return to an AK. Iron sights are "easy" to reach out to 200yards for near on anyone...even I can do it.