So recently, 2 of my friends have experienced really bad artifacting in custom maps, and they both have the same card: GTX 660. Would this be a driver issue or something else entirely, because they say it gets to bad to the point they can't see anything in the game anymore.

One thing I'd like to ask: My friends and I built the acid gat converter, but we were never able to actually get a blundergat.  Is it unlocked through the map somehow?  We spent 3 hours playing and we never even saw it go through the gun rotations in the box.

Everytime we get to the part where you kill zombies near the flaming stones the map lags horribly for 5-10 seconds at a time, and does this for the rest of the game, and it's whenever we look at the zombies.