SPMBT AMMO LIMIT

[low_bidder]

On the 'preferences' screen, there is a toggle (button) that sets the ammo supply to limited or unlimited for that battle. It's not called that, I forget what it is called. In one position your weapons are like Hopalong Cassidy's 6 shooter. They never run out of shots. In the other position they have the rounds available to them that is indicated on the weapon line of the Unit screen.
If you mean in battle reloading, you have to have an ordinance section(ammo resupply).
You reload a unit by being in the same hex as the ordinance unit. ('shift' and click on that hex) or in 1 of the 6 hexes next to the hex the ammo resupply unit is in. Helios MUST be landed.
OFF map units can not be resupplied. What you can do is turn off a weapons slot to conserve ammo. You click on it in the unit screen and it turns blue and won't shoot. This is called blue-lining by some of us old farts.You can do it to any unit, not just off map arty and air strikes.
It's useful for when you are running short on main gun HE and are killing infantry. Blue line the main gun and you can point and click with you machine guns. Don't forget to turn your main gun back on for Op fire shots. It is also useful for keeping an enemy armored vehicle suppressed when you are trying a bump and run.