This is kind of a conundrum that may not have a solution.
1. If I need a car gun, I'm probably in a place where there is a good chance of the gun and/or the car being stolen if I leave it around.
2. If I am in the car when I need the car gun, there's a good chance I will need it much faster than I can reach it, especially if I have taken elementary precautions against #1.
3. The most physically convenient and powerful car gun would be a short-barreled rifle or shotgun, which brings NFA compliance issues and drastically increases the financial cost in the event of #1.
4. Leaving aside the ethical concerns of having a loaded firearm in a place where theft is highly possible, the most financially convenient car gun would be one that was inexpensive and wouldn't be missed. However, that means that it's also not likely to be a more effective weapon than the pistol I am already carrying.
So I guess when it comes down to it, if after considering all of the above, I was still set on getting a car gun, I'd have to say a Kel-Tec Sub-2000 or a Hi-Point 9mm carbine would probably be it.