Metal Shed (<i>Land_Metal_Shed_F</i>, located at 028025 for example) is extremely difficult to penetrate with bullets. While it appears to be made out of 2mm thick metal plates, a 9mm bullet won't go through it at all and a 6.5mm bullet will have its velocity severely affected, drastically reducing its damage.
http://www.youtube.com/watch?v=rOLiaEhRr9E