Haven't read the whole thread, so I'm sorry if I'm redundent or if these have allready been answered.
First: What are looking for? What is your definition of a car gun? Are looking at something that you can shoot through a car and hit the driver, something you can carry in YOUR car, or something that can mess up the engine enough to put a vehicle out of commision?
If you're looking for something that can be used to penetrate into the "cockpit" of the car, then I highly recomend whatever gun you happen to have. Contrary to public opinion, a car is not cover. You can get behind the axle or the engine and use that part of the car as cover, but most of the car is going to be too thin-skinned to protect you much in a gunfight. That goes for the bad-guy too. If he's using a car door for cover, he' doesn't really have much cover. Check out the Box of Truth(
http://www.theboxotruth.com/docs/thebuickotruth.htm) they have a section on shooting cars.
If you're looking for a gun that can be carried in a car, you have a lot of decissions to make: are you going to carry on your person, or is the gun going to be in the door or glove box? How are you going to carry on your person (SHoulder holster, crossdraw, hip, IWB, ankle). If your gun is off body, are you going to leave it loose in a pocket holster, or fix a paddle holster to soemthing in the car? My car gun used to be an H&R single shot shotgun, but when I got my carry permit, I started carrying a S&W .38. I like it because I can put it in an inside jacket pocket and still reach it when seated, or throw it in the glovebox.
Are you looking at shooting the engine to disable the car? I don't have much knowledge in that area, and even less experiance, but I think one of the members here stopped a car with a .41 Magnum. Personally, I'd be looking at a long gun: either in the .308 or 12 guage ballpark. I don't know what the minimum calliber for that is, but I suspect either of those would get the job done. If not, well then, I guess it wasn't really a "gun problem" to begin with.
Hope this helps,
Chris "the Kayak-Man" Johnson