I did originally have plans to make the units have limited ammo, and have to resupply but there are two major issues with it:
- the feature isn't easily supported in existing code/mod API, it will most likely use a lot of custom coding and use more CPU %
- with current path finding issues and load on CPU, I feel it's in the overall mod user's gameplay experience and happiness to avoid simulating ammunition because it will overly complicate the system.
If there was a nice simple way to do it which doesn't turn your base into a mess of back-and-forth robots, combined with a better way for the mod API to handle/simulate it without destroying people's processors, then I will try to add it. currently no plans, and probably not going to be good enough mod API features in 0.17 to do it yet either.