Think it thru ...
If you have more than 0 ammo_available , add 18 to current_ammo...no good (ammo_available becomes less than 0) , > 18 working...
Solution :
If you have more than 0 ammo_available ,
If ammo available is < than 18
add ammo_available to current_ammo
set ammo_available to 0
Else
add 18 to current_ammo
substract 18 from ammo available

Too lazy to correct your script You'll have to do it yourself