I thought the story took place in Europe? :P
Is the ammo sold through vendors, or unsupervised through a machine? Because
after all, ammo isn't candy...
Logical placement is also a concern. I don't think autoguns will buy their own
ammo, and the people who maintain them will probably bring some ammo from a
storage somewhere. Though I suppose it could be used for humoristic effect: "Oh
snap, I forgot the ammo for the autoguns! Got a dime?"

Yes, the vending machines are taller than the walls, but so are many obstacles.
Even doors are a little bit taller. I think it's fine. :)