It's hard to explain why something as seemingly simple as a cannon or even a Gatling gun would need as many prototypes as we and you have made. I'm sure we both try to work out as many design elements as possible on paper first, but that is never perfect, changes must always be made, hence the numbers of prototypes. Your Proto #2 sounds great; those would be significant improvements! And #3 sounds like the Ultimate to us, without going to cartridges and all the licensing requirements that go with manufacturing and selling cartridge guns.
Mike and I have our doubts about pumpkin enemies, however, we don't think they have enough liquid inside to respond to the hydrostatic pressure caused by a ball's entry, which would cause them give up their nefarious and truly evil thoughts, like bursting mellons do!
We will follow the development of #2 with great interest,
Tracy and Mike