If it's a motley crew of survivors, I think it'd actually be more realistic to [i]have[/i] a gun expert/military dude or two, since I'm expecting they'd have firearms at home as well as experience in dangerous situations that would enable them to wield their weapons with great effectiveness and get out of hairy situations. 

Military dude > katana wielding expert in my opinion.

I think the best way to curb OPness would be to simply set it out in the rules so that players know right off the bat what you're looking for rather than having to think up ways to hamper said gun experts during the actual game. Like in my group RP here on RPG, the GMs set what types of weapons would be allowed, which would be rare without a good reason, and which ones were just a big fat "No".