Every armed vehicle carries a Re-arm action. This covers an armed jeep, an APC, and a helicopter gunship alike.
It is one action and one full restock. Every turret magazine and every rocket pod on the vehicle comes back full together. There is no way to refill only part of a vehicle.
A re-arm spends storage before it spends money. It checks the storage the vehicle carries first. It then checks any storage within twenty five metres that you are allowed to draw from, for the magazines the vehicle weapons take. A re-arm spends anything it finds there, instead of buying it.
When storage covers the whole re-arm, the action reads Re-arm (from storage). It costs nothing, and it works anywhere. A crew that hauled its own ammunition is not tied to a base.
Only what storage cannot cover gets bought. The price is that same share of the full price, so half the magazines covered means half the bill.
That bought part is the only part with a place attached to it. The vehicle must stand at a built Helipad or a built Garage, at a base the resistance holds. It can also stand within a hundred metres of a deployed FOB. Anywhere else, if something still needs buying, the action shows blocked. It then names the helipad, garage, or FOB it wants.
You can import ammunition for a mounted vehicle weapon at a port, and move it from one storage list to another. It never comes out into your inventory: taking it out reports it as missing, and it stays on the list.
That is exactly where a re-arm looks for it. A crate of it on the list of a truck parked alongside counts for as much as a crate on the vehicle itself.