Looking at the the bullet holes in the car pictured, it seems to me they are more rounded as if fired into the car from directly from the side and not shredded like bullets or cannon shells would do to an airplane. In that case, I'd sand down the inside of the plastic to a thin layer using Reggie's abovementioned technique. Paint the car and when the paint dries, take a round, pointed probe and punch through. Some of the paint may chip or flake. If it does, you can flake it off a little, but not much, since there doesn't appear that very much paint flaked off in the pictures. Then if you want to rust it, take a some steel wool, soak it in water for a few days until it rusts; crush the rusted part into powder and apply with a small brush around the holes. Then seal the rust powder under a coat of Future or other topcoat.
That's my idea, I hope it helps.