Armor in a car doesn't mean bullets will ricochet on the paint. They will still penetrwte the car body but are stopped by internal armor if the caliber isn't bigger than the stopping power of the armor. The angle of attack also has a big impact. A bullet hitting the armor with 90° on the x and 90° on the y axis is the same as an armor piercing bullet (those bullets create their own 90°x 90°y surface by melding a piece of softer metal onto the agled armor). And converting a car capable of stopping 50cal AP rounds is really hard. Most armor in cars is to stop lower caliber rounds as the chance of people with military grade high calibers and AP rounds is rather low.