Best way to implement a Wave Motion Gun as a class feature?

[Greywander]

I'm working on a homebrew class that transforms into, well, this (https://tvtropes.org/pmwiki/pmwiki.php/Main/MilitaryMashupMachine) (with subclasses for the amphibious submersible battleship (https://tvtropes.org/pmwiki/pmwiki.php/Main/ShipOutOfWater), the burrowing tank (https://tvtropes.org/pmwiki/pmwiki.php/Main/DrillTank), and the sky battleship (https://tvtropes.org/pmwiki/pmwiki.php/Main/DreadZeppelin)), and wanted to include a Wave Motion Gun (https://tvtropes.org/pmwiki/pmwiki.php/Main/WaveMotionGun) as one of its features. At this point, I've kind of given up on properly balancing the class, and the best I'm hoping for is that having one of these in your party is still fun for everyone (and I've taken steps to help promote that).

So my question is how to best implement a wave motion gun? I mean, I could make it a once per long rest ability, but that seems boring.

Currently, I have you spending hit dice to feed into an initial pool, then you double the pool by spending subsequent actions to charge the gun up. You can quick-charge the gun by spending more hit dice into the initial pool, but then you can't use the gun as much, and you might want those hit dice to heal later. If you fail to spend an action to either charge or shoot the gun (e.g. you get paralyzed before you can shoot), then all the stored energy explodes and you take all the damage in the pool. When you shoot the gun, it creates a line AoE and offers a DEX save for half damage, but you also take half the damage. So while you can charge it indefinitely, eventually the damage will be so high that you kill yourself (could make for a great heroic sacrifice). I think I'll have to playtest this to get a good feel for the balance between the cost and the power of the ability.

What I'm more concerned about is the size of the AoE. Currently, it's 300 feet long and 10 feet wide. 300 feet long is a decent range for medium sized creatures on foot, but it starts to look a lot less impressive when it comes to ship-to-ship combat, or equivalent (e.g. fighting an enemy airship, or a dragon, etc.). Iowa class battleships apparently have a range of up to 24 miles, though to be fair we can't really compare a modern battleship to a medieval fantasy battleship. Dreadnoughts (the predecessor to battleships, and the namesake of this homebrew class) are tamer at typical ranges of 1 to 2.5 miles (Edit: this is wrong, those were for earlier ships; dreadnoughts used 12-pounders with an effective range of more than 5 miles). Usually making an AoE bigger will make it a lot more potent, but for lines I feel like making it longer doesn't usually help, only a wider line will significantly increase the number of targets you can catch in it. So do you think it's "safe" to extend the line to allow the gun to be used at longer ranges? How far do you think it should extend?

[Durazno]

One possibility would be to give the character or attack an anti-personnel mode and an anti-material mode. The ship-scale wave motion cannon could then be a weapon for special occasions, while still giving the character a weapon they can use more regularly.

Imagine a pool of, say, 20 charges. You could spend one charge to fire your main gun in a beam that rakes down a dungeon corridor and scorches a bunch of goblins, or five charges to clamp yourself to the deck of an airship and fire an explosive death ray at an airship 2.5 miles away. The anti-material beam could have aspects that would keep you from wanting to use it in tight quarters - backwash, rendering you immobile, forcing you to recharge after, or something else.

[Greywander]

You can already produce a gun turret out of your body as a natural weapon, so you do have something you can use regularly. I also have an AoE you can use where you shoot all your cannons at once, where the size of the AoE depends on which vehicle form you're using (there's a minimal damage increase for larger sizes, but the main thing is hitting a wider area). So I think the "use more regularly" thing is already covered by those.

Also nevermind clamping yourself to an airship, this class lets you literally become an airship. You don't unlock the gargantuan form until tier 4, and even then gargantuan is kind of small (typical ships will be 40+ feet, some longer than 100 feet; 20 feet is tiny by comparison), though you can always fluff yourself to be as big as you like so long as it doesn't affect how much space you occupy/control in combat.

I made some tweaks to the wave motion gun. I gave it a base range of 600 feet. You actually unlock your huge form before you get the wave motion gun (both are tier 3), and the huge form doubles your range for your turrets and such, so I just extended that to include the wave motion gun. This means you can hit a range of 1200 feet, but only while in your huge form; in a large or smaller form, you can still snipe from 600 feet away, which is enough to hit a longbow user at their max range. In tier 4, you unlock your gargantuan form, which triples your range (so up to 1800 feet), and then one level later you get a feature that doubles your range all the time (so 1200 feet while large or smaller, 2400 feet while huge, and 3600 feet while gargantuan). This seems like a decent compromise, requiring you to assume your largest form to get maximum range, and if you're shooting something that far away then you will most likely have room to take your largest form.

You may have noticed that a major balancing element will be the DM forcing you into confined spaces like a dungeon where you have to revert to a smaller vehicle form. As I said, I've pretty much given up on balancing this properly, but while in your medium form you should be comparable in power to the rest of your party. In larger forms you get slightly stronger, but your main power boost comes from having other people crew turrets and such, which in turn helps involve the rest of the party if e.g. you engage in something like ship-to-ship combat. So it might not be balanced, but hopefully it will at least be fun. And turning into a humongous mecha to wrestle the tarrasque to the ground while your party blasts it with gun turrets does sound pretty fun.

[Kerleth]

To me, a wave-motion gun has the charge up sequence as a central part of it's identity. How about having the damage increase for each round of combat you don't use it. Example: If you use it on first turn it's xd4, on 2nd is xd6, 3rd is xd8, 4th is xd10, and 5th+ is xd12. The (x) part increases with class level to keep it at an appropriate amount. This encourages the player to use it as a big flashy finisher while not being overpowered and keeping them from turning into a super sniper. It's also a niche that not really anything in the player's handbook covers, giving it some unique appeal in playstyle.

[Greywander]

But when would it start charging? Couldn't it charge between encounters? This would then lead to the first shot always being at full strength in every encounter, making it more of an opener instead of a finisher.

This is something I've already accounted for in my design. You have to spend an action to charge it, so it doesn't just passively charge while you're not using it. Once you start charging up the cannon, you must either keep charging it or shoot it. So you can't pre-charge it, except when using it in an ambush. You also take half the damage, so using the wave motion gun frivolously will only hurt yourself. There's also no cap on how much you can charge it up, and each action used to charge up will double the damage dice, meaning it ramps up fast. If you mistime it and end up charging for one turn longer than you intended, you could easily end up killing yourself.

Part of the appeal of a wave motion gun is that it is overpowered, but generally they're also a bit awkward to use. So high damage is key, but the charge up time and the backlash damage help reign it in. I'll have to playtest it, but it looks like what I currently have should work pretty well. Due to the doubling with each charge up, the gun gets exponentially more efficient in terms of action economy (i.e. damage per action spent) the longer you charge it up, so it will definitely push you toward waiting until it's charged up enough before you use it.

I'm open to other ideas, though. For example, I could take your idea and combine it with the idea that once you start charging it up, you must either keep charging or shoot it. Maybe instead of taking half the damage dealt with no cap on charging, there might be a cap, and you'd only take damage if you exceeded that cap and not from just shooting it normally. So what I have now is just one way to handle it, there are definitely other approaches I could try.