There is ammo all over the place, and so are guns. I like to role-play that they gunshots attract the infected in the surrounding hills, so my character avoids using the gun - and only in dire situations.
Rarely (I have only had to do this once) if there is a lot of commotion going on - for example The Wild Territory; lots of people, and lots of gunshots, I will wait a day or two in-game, and next time there is a whole lot of commotion I will send in a horde. It makes sense.