The zombies in Bio-Annihilation II are very persistant if they want to get to you. I made a bunch of wire-fence barriers around a city that I then physics-gun-locked. They were supposed to be breathing room from the zombies so you could shoot them and then find an area to recooperate. The only way to get past them was to jump over them via vaulting from vjanip.
Start spawning in the zombies and:
They tear through the wire fences like butter.
Well, there's my plan out the window.

What would you suggest as very thin barriers to shoot through they can't just knock down?