Okay, from what I gather the consensus for the ballista-type weapon, it's probably going to be a medium one. Machiavelliest, I've been toying with your idea of a pericing/fire weapon, and here's what I've come up with:
Salamander Deck Gun
Range: Short to Medium-Short
Damage: Piercing, Fire
Clip size: 3
Effect description: The weapon shoots one large-ish round at a time. This solid round does pure piercing damage. After a certain amount of time, the round detonates, showing a forward facing, cone-shaped area with fire damage.
Why would it be fun to use?: It would take tome for a good gunner to learn just where the piercing ends and the fire starts. It would be fun to learn the ins-and-outs of the gun. Plus, it encourages coordination, as a good captian could work with his gunner to use the fire damage to disorient the opponent, then go in for the kill with the solid shell.
What does it bring to the table? It will be unique in the fact that it allows fire damage to be dealt at medium range. It also offers some flexibility in its use. I can see it being used on all ships. A good squid can use it to serve the gatling and flamethrower roles. A junker could take it instead of gatling guns for more versatility (at the expense of reliability). Goldfish could use this weapon to complement almost any main gun well. For pyramids, the option is there to mount one on the front if they liked flamethrowers on the front, but wish they had a little more reach. Spires, well, spires don't really like short range, so this gun offers little to them, but I can see this gun being used on the rear or side of a galleon.
Why would it be fun to play against?:
Captains would have to weigh closing in and suffering hull damage, versus staying out of range and maybe suffering fire damage. It offers another series of choices that the captains would have to make. Engineers would have more to do at those awkward medium-ranges, by putting fires out. It would also be fun to watch the spectacle with every shell that detonates into a puff of fire.
So, that's what I got. Thoughts?
