The AK47, in the games I played, has been a big contender of weapons to loot. TPG-1 and Remington 870 are pretty good as well.
A good portion of the game is to hunt for supplies. Remember these places, since the locations aren't randomized, except open street supply crates. If you found enough, go hide. However, move once in a while to locate health packs, since the mode is designed to damage you overtime (think poison gas residue).
If you are getting into a fight, hide and heal to about 3/4 of your max health before taking them on.
Also make use of the gas mask. It'll allow you to walk into the encircling poison gas without taking significant damage. Can't say if it expires after a while inside the gas clouds. Never tried.