Making the powder keg a stackable object and removing gunpowder altogether doesn't sound like a bad idea. However, I can foresee one problem: The player makes a powder keg, and uses it to shoot a few shots with his cannon. Then he takes the keg out of the cannon and wants to make some lead shot with the gunpowder. How does the game handle creating x-amount of lead shot when the powder keg is missing some powder?

You could make a calculation to determine how much leadshot is made based on the amount of black-powder left in the barrel, but because you cannot make a fraction of a lead shot, this would lead to wasting blackpowder (unless you got 12 lead-shot from one powder keg...).