I see no reason why you couldn't armor up a car. Even places like Kali and Chicago likely wouldn't care. But i don't know about mounting guns on it. I guess if you're allowed to open carry or have a gun visible in your car, then it should be OK. but I would make damn sure the gun is mounted on tight and unloaded so it doesn't get lifted when you leave your car parked somewhere.
MMmmm, getting a HUMVEE and mounting a .50 on the roof would be just the thing to keep other angry drivers in their cars and away from you
Just out of curiosity, any particular reason you trying to do this? Sounds like a pricey project.