Something I noticed that might save you some grief later...
Both gun cars, guncar and flakcar will crash the game if you put them in a terrain and kill them.
The first shot kills the gun but the second shot, which you expect to destroy the car, crashes the game. Oh, and if you put both cars in the terrain and kill either gun, the other gun dies too even if owned by a different field.
I haven't tried every combination of property type.