As the developer you will have to decide where to store the values and what to name them, but the logic is as follows:
You will need two variables weaponAmmo & storedAmmo.
weaponAmmo = 10; storedAmmo = 20;
Events would look something like this:
Condition: On Fire; Action: subtract 1 from weapon ammo;
Condition: On Reload & weaponAmmo is less than 10; Action 1: Set storedAmmo to (storedAmmo-(10-weaponAmmo)); Action 2: Set weaponAmmo to 10
Start there, you will need to adavance the algorythm for when storedAmmo is less than ammo need to fill it gun.
By the question I suppose you are pretty new to this stuff, stick with it and get used to creating your own logic, use the manual.