A ship which is a living hell from a distance to any other ship-class (not just fighters) but it's also very vulnerable to every class of ship from up-close. Super weapon, strategic nightmare. Position yourself, fire away then change your position or just run because the whole squadron is after you.
It'd be also great for perimeter protection of bases. Don't think of it just a stationary weapon which just keeps on firing. Just like any other weapon, it ain't perfect. You fire away in a crowd, you take away only shields. You have to wait 5 seconds to fire the next shot at the enemy who now has shields down and who is already dispersed or heading at you.
Nitram: I wouldn't like to mix this with a GB (nor any other class). I just used idea of gunboat as reference to size. Simple: a new class of ship. Most people would find it boring to operate but I'm sure there are those who'd use it all the time.
Rodent: You said it.
McNeo, Dragonego: Misuse could be controlled. Special weapon could be strictly controlled. Every piece of artillery would have to be registered and/or approved by faction who owns it.
Indie artillery? Why not make that one as well but which deals less damage.
Schlurbi: there you've said it as well but I wouldn't agree with the battleship part. Let's have a break from capital ships. This is why this would be cool because it's a new class of ship. It's not like I invented something new and unheard of before but we definitely need something to refresh the battlefield.
Ironfoot: mines with timer is an excellent idea. Also adding proximity mines would fit this profile as well.
Cabulb: I have to disagree, Cab. Cruisers are simply light battleships which can more quickly react on a battlefield than the battleships itself. They're multi-purposed if you ask me. You've probably referred on pulses and mortars which can do damage from distance by being mounted on a trusted ship but artillery is something entirely different in this case. It is not multi-purposed, it is simply and nothing else except a long range gun which can do damage on wider area.
On the other hand, mounting this weapon on cruisers/battleships is a good idea but those would be FAR LESS powerful than a actual artillery piece i'm suggesting. At least that sounds more balanced. I guess.