Availability of ammunition, or the ability to manufacture it reliably is obviously something that will shape the armies and combat doctrines of your faction. All That Remains being a post-apocalyptic game, reliable supplies of ammunition may be hard to come by.
|Unless you're playing in The Walking Dead universe where everyone is apparently resupplied by the USAF on a nightly basis...|
Whenever a unit which includes one or more non-primitive firearms fires it's weapons, it rolls an ammo dice, which with a roll of a one indicated that that unit has run out of ammo - for it's modern weapons.
Units can be resupplied with logistics points, which are purchased during your force selection, and are spent to resupply your troops. If you run out of logistics points, your squads will start to run out of ammunition.
It should be noted that costs for logistics points "ramp up", i.e. the first one will cost X, the next will cost 2X the next 4X and so on, so eventually buying another logistics point will become uneconomical, where that point is, is up to you.
That's all there is to it really.
I haven't put that much work into it yet, but I'm currently leaning towards not changing the cost dependent on setting level, the cost of guns themselves should be enough to incentivise or deincentives their use.