CZ2D: Rules of the Game no. 1:
'If you act like a dick, or start pretending to be Rambo, you WILL die.'
I think that sums up one of the main themes of CZ2D. It will be brutal, beyond Red Orchestra kind of brutal. So if you start running around shooting everything, you will end up killing yourself, either by the sound of the shots attracting a horde of zombies, or using up all your ammo or some such thing. However, if you're smart and you conserve your ammo and keep your wits about you, you'll be able to survive pretty easily and be dig out a good little virtual life for yourself in the zombie-infested wasteland.
As such, this is my design for the ammo-counter:
|MS Paint mockups ftw!|
So, this shows several things.
First, notice there are 5 bars. These represent the up to 5 magazines the player can carry for each weapon (if the weapon uses magazines), bar 5 is coloured dark grey because the player hasn't found a 5th magazine yet (they'll be fairly common though, most dead survivors you'd come across would have 2 or 4 magazines for the weapon they are carrying).
Second, notice that not all of them are full. This is where the brutal realism comes in, at some point in the past the player here has either swapped out a nearly-full magazine for a more full one or just not had enough ammo to fill the magazines up. Pressing the reload button (most likely R key) will make the player character swap out the current magazine for the most full one (not sure what to do if the current magazine IS the most full one, perhaps R should have a different function). Holding R, however, will make the player start refilling their reserve magazines, first with loose ammo from their inventory which is limited only by how much they can find and carry, and then with ammo from other magazines, taking rounds out of the emptiest ones to try and fill the others. This means the player isn't actually limited in the amount of ammo they can carry, just in the amount they can use at once before having to find somewhere to hide and reload.
Third thing to notice, there's a weird symbol top-centre and bar 4 is coloured light grey. This is because the player is currently using armour-piercing rounds (not the wisest choice against zombies, but perhaps useful if you want to hit them through something) and bar 4 is filled with a different kind of ammo. Not sure what key would be to change ammo type, maybe T.
So how does this tie in with Rules of the Game no. 1?
If a player starts being a dick (read: FPS gamer) and running around guns-blazing, constantly reloading, they're going to attract an awful lot of attention as well as end up with 5 almost-empty magazines (as the average FPS player is quite unable to cease firing long enough to reload them), thus they will die as the horde absorbs the last of their readied ammunition and proceeds to tear them limb from limb.
As always, feedback would be appreciated.