Left 4 Dead 2
Well, technically, they're just immune to the virus, and at least statistically, the chances of having a veteran amongst them is fairly low.
Rochelle is the only one I can't really see as believable. Ellis pretty frequently mentions having a love of blowing stuff up, and was probably raised around firearms. Coach might not be in tip top shape, but being involved in sports actually teaches you a pretty good bit about violence - he knows how to take people (or infected, for that matter) down relatively efficiently. Nick is a conman, and the other characters mention that they aren't surprised that he's good with a gun, so it can be assumed he might have experience with firearms as well.
Though, if Zoey was a viable character, Rochelle technically could be, too - she probably heard what the government suggested survivors do firsthand (or at least secondhand) and could've produced violent TV shows, for all we know.
On topic: Bill for L4D1, Ellis for L4D2.
In Zoey's backstory, it mentions that she is a horror enthusiast. She watches horror movies, reads books, and focuses on zombie folklore. She's more believable than Rochelle.
My problem with the L4D2 survivors, is that they can pick up and instantly master any firearm they come into contact with. It breaks the immersion a bit. In the first game, it makes sense that they have experience with all the weapons. They are all fairly basic.
Take Coach for instance. Yes, he was involved in sports. Yes, he knows violence. But knowing how to take someone down doesn't equate to knowing the intricacies of all the weapons you see them using.
Which brings me to my next point...
For the L4D2 survivors, barring Nick, the experience just isn't there. There's a big difference between knowing how to do something, and actually being able to do it. That was my problem with Zoey, and it's my problem with Coach, Rochelle, and Ellis. Even if Ellis was raised around firearms, he most likely wasn't using them against people, seeing how naive he is.
That's been the problem with a lot of Valve games. Their stories are good, but the characters in them don't fit.
I do see your point, but the characters really haven't mastered any of the weapons. I mean, just look at the crosshair - it's HUGE. Their accuracy is horrible, and to even hope to hit someone far away you have to stop and crouch down.