I still recommend getting a copy of Omega World. It is easy to take out the high-tech (ray guns) and leave in the stuff that currently exists (firearms). Also, if there are no mutations, then leave those out, too. I think some kind of "powers" (magic, psionics, the Force, etc.) are what make RPGs compelling for players; but they can certainly be curtailed or eliminated altogether.
You may also be able to make your game using what is in D&D 3.x. The DMG has a section on modern weapons. I think the feat is Exotic Weapons-Firearms. The rest is pretty portable. There are simple rules for dynamite, shotguns & full-auto (maybe even grenades). (If you want more detail, take it from the d20 Modern SRD.) You could even use the NPC classes, warrior & expert in particular, for the PCs. From the PHB, the fighter (maybe an ex-soldier), & rogue (a skilled survivor) could be used for a PA game without magic. You might even drop in the barbarian (a savage) and the monk (a wanderer). The best bonus of all is that you & your group already know the system, so there is no new material to master. All you have to do is use the game you're already playing in new ways to make the setting come alive. I've considered this many times.