TFL doesn't do SHTF stuff. So, only the 'trunk gun' concept can be addressed.
I don't leave valuable items unattended.
That means never leaving guns in a vehicle, when in civilization. (It often can't be avoided when hunting.)
I could lock something up in a vehicle, via a chain, safe, etc. But that defeats the purpose of the 'trunk gun'.
So, the answer is, "whatever I'm carrying on my person" ... which isn't going to be a rifle.
If I were the type of person that was willing to risk leaving a rifle in the 'trunk', it would probably be an AR pistol of some kind*. More than likely, chambered for .223/5.56. If, for whatever reason, I felt that .223/5.56 wasn't enough, I'd jump to .458 SOCOM. Runs like a rifle. Shoots like a rifle. But smaller, lighter (usually), and easier to keep out of the way.
*(An SBR would be better. But then you're going to have to answer some unpleasant questions if it were to get stolen; and may make yourself vulnerable to civil suit if the weapon is used to injure or kill someone, driven by the perceived negligence of leaving an NFA item in your vehicle.)