How about ammo trucks?

[Irinami]

I'd recommend some ammo trucks added to your core eventually. I mean, if you're going to use them a lot, you may as well add them. If they're permanently attached... then permanently attach them, right?

As I understand it, ammo is reloaded in the left-to-right order you see on the unit info screen: HE, AP, HEAT, APCR (not positive on those last 2, too lazy to open it up and check).