no-gunpowder cannon

[BeefGood]

The party has a ship and I would like to make cannons an option according to this sentence in the DMG (p. 255): “In a campaign without gunpowder, a cannon might be an arcane device built by clever gnomes or wizardly engineers.” The sentence caught my eye because the party includes a gnome wizard. I’d like to give him something neat to do. Gunpowder v. no gunpowder doesn’t matter that much.
I’m not sure how to do this and I would appreciate others’ thoughts. Here is what I’ve come up with so far:
The wizard would craft the cannon according to the rules for crafting magic items in XGtE. The cannon does 8d10 damage which is the damage associated with a 5th level spell (DMG p. 284.) A 5th level spell scroll is rare (DMG p.200). Crafting a rare magic item requires 10 workweeks and 2,000 gp (XGtE p.129). Partnership with a weaponsmith/armorer and a carpenter would be required. That would bring the time down to 3-4 workweeks but these people would have to be paid.
XgtE on the same page says that an exotic material should be required to craft a magic item. The magic of this cannon replaces gunpowder so some exotic material related to gunpowder, explosions, or force. Not sure what. And/or maybe a quantity of adamantine to line the barrel.
Many magic items have limitations on the number of uses per day. I don’t have an idea of the appropriate number of uses and I’m not entirely sure a limitation is needed. The resource cost in firing the cannon would be an action to load it, an action to aim it, the wizard’s action (or maybe a bonus action) to fire it (by speaking a command word), and a cannonball. Or, using the ship rules from Ghost of Saltmarsh, assuming adequate crew, it would require a ship’s action, the wizard’s action/bonus action, and a cannonball.
Thoughts on any of this, or other approaches? Thanks.

[Garfunion]

Your the DM. You can hand wave a lot of the stuff you find clunky or annoying.

Another question are you just trying to avoid chemical explosions as weapons?

This may be a good read for you. It is from Eberron.

http://keith-baker.com/crossbows/

[ZorroGames]

In my fantasy war games I have seen Steamed powered guns; methane explosion chamber guns; from a series of books, magical compresssed explosive pellets released by contact with a drop of water; and bound air elementals powering the shot.

[Clistenes]

What about a steam cannon (https://forums.giantitp.com/showthread.php?571567-Got-a-Real-World-Weapon-Armor-or-Tactics-Question-Mk-XXVII/page44)?.
You coud use Heat Metal on the barrel, then pour water inside...

[micahaphone]

In world war 1 France used some spring loaded mortars for a bit. Or you could invent a magically infused potion that explodes 30 seconds after being exposed to air. Pour in the barrel, load, aim!

[Garfunion]

What about a steam cannon (https://forums.giantitp.com/showthread.php?571567-Got-a-Real-World-Weapon-Armor-or-Tactics-Question-Mk-XXVII/page44)?.
You coud use Heat Metal on the barrel, then pour water inside...
I like this idea

[Balyano]

What about a steam cannon (https://forums.giantitp.com/showthread.php?571567-Got-a-Real-World-Weapon-Armor-or-Tactics-Question-Mk-XXVII/page44)?.
You coud use Heat Metal on the barrel, then pour water inside...

put the water in first and cap the end with an object that will break when the desired amount of pressure is achieved. THEN you cast heat metal. If you are short on magi build a fire around the water bulb portion and wait till it goes boom. A lot slower without the spell caster support.

Could enchant the bulb to cast heat metal on itself.

Had a steam engine that had a plate of heat metal and a coil of chill metal to power it and regenerate the water from the steam.

[HappyDaze]

If you want to control the use somewhat, have the cannonballs be the (expendable) magic items and the cannon is just the (mundane) device required to direct them at a target.

[Knaight]

Pneumatic cannons are very doable; in general the restrictions on cannon development were less generating an explosion and more material strength - cannons need to be light enough to move, but tough enough to withstand their internal explosions. D&D magic can push both those factors.

[JackPhoenix]

Just make a wand of Catapult spell.

[Temperjoke]

Just make a wand of Catapult spell.

Or a wand that casts any offensive spell, really, you just make it bigger.

[Tvtyrant]

The party has a ship and I would like to make cannons an option according to this sentence in the DMG (p. 255): “In a campaign without gunpowder, a cannon might be an arcane device built by clever gnomes or wizardly engineers.” The sentence caught my eye because the party includes a gnome wizard. I’d like to give him something neat to do. Gunpowder v. no gunpowder doesn’t matter that much.
I’m not sure how to do this and I would appreciate others’ thoughts. Here is what I’ve come up with so far:
The wizard would craft the cannon according to the rules for crafting magic items in XGtE. The cannon does 8d10 damage which is the damage associated with a 5th level spell (DMG p. 284.) A 5th level spell scroll is rare (DMG p.200). Crafting a rare magic item requires 10 workweeks and 2,000 gp (XGtE p.129). Partnership with a weaponsmith/armorer and a carpenter would be required. That would bring the time down to 3-4 workweeks but these people would have to be paid.
XgtE on the same page says that an exotic material should be required to craft a magic item. The magic of this cannon replaces gunpowder so some exotic material related to gunpowder, explosions, or force. Not sure what. And/or maybe a quantity of adamantine to line the barrel.
Many magic items have limitations on the number of uses per day. I don’t have an idea of the appropriate number of uses and I’m not entirely sure a limitation is needed. The resource cost in firing the cannon would be an action to load it, an action to aim it, the wizard’s action (or maybe a bonus action) to fire it (by speaking a command word), and a cannonball. Or, using the ship rules from Ghost of Saltmarsh, assuming adequate crew, it would require a ship’s action, the wizard’s action/bonus action, and a cannonball.
Thoughts on any of this, or other approaches? Thanks.
Shannara had light cannons with magic gems that emitted lasers, which I think is pretty magic-punk. Another possibility would be dragon scales or phoenix feathers which get used up when fired, so alliances with or hunting of magic creatures is necessary for military to stay functioning.

[Vogie]

Huge contraption that works as a Mounted Wand of Delayed Blast Fireball? 12d6 has a similar damage to 8d10, with a slightly lower max damage.

[Sigreid]

Enchanting a cannon shaped device with a form of heat metal for instant steam power. Or binding ice and fire mephits/elementals into it to instantly produce steam could be fun.

[Clistenes]

put the water in first and cap the end with an object that will break when the desired amount of pressure is achieved. THEN you cast heat metal. If you are short on magi build a fire around the water bulb portion and wait till it goes boom. A lot slower without the spell caster support.

Could enchant the bulb to cast heat metal on itself.

Had a steam engine that had a plate of heat metal and a coil of chill metal to power it and regenerate the water from the steam.

The model mythbusters built had the barrel/chamber heated first, then water injected...

[BeefGood]

Thanks all for these excellent ideas. At the moment I’m leaning toward enchant-the-cannnonball, but really, given the concept of magic as a capability-extender from that Eberron article, all of these ideas are workable.
In fact I should probably just back off and see what the party comes up with.

[CapnWildefyr]

three thoughts come to mind:
1. Back in the "good ol' days" there was no gunpowder. Instead it was a magic item called 'smokepowder' which was, you know, the same thing. Except that as a magic item it can't be conjured.
2. Make a hollow metal tube that uses a L3 catapult spell, and shoots iron balls. Load one end, command word or button, whoosh! Fires the ball out the other end like an EM accelerator. Maybe 2d4 uses per day (the roll is for each cannon, forever, so some might be better than others--just like medieval cannons), takes x rounds to recharge (or recharge on a 6 or after 1 minute). Best not to allow a high reload rate--the DMG says 3 actions but a crew could do that in one round (or they'd argue for it!)
3. IRL a guy made a steam-powered machine gun. Problem was it fired superfast but could not be aimed and needed a steam engine and a LOT of time to charge I think.

[Sigreid]

You could always just give it whatever stats you want and say it's enchanted with a variation of the Catapult spell.

[Yakk]

Force Cannon

6d12 damage (siege: 2x vs objects) on one target.

To fire this cannon, you place an enchanted plate within a slot, and close it. Then you touch the sigil on the cannon. A lance of force damage shoots out 1 mile.

Using the Force Cannon takes the following 4 actions, in order.

Reload and Range: Intelligence check.
Adjust: Strength check. At disadvantage if not done on the same round as Range.
Aim: Dex check. At disadvantage if done on the same round as Adjust.
Fire: Charisma check. At disadvantage if not done on the same round as Aim. You can expend a spell slot to add a bonus to this check equal to the slot level, and also increases the damage by 1d12 per slot level. Cannot be repeated; the gun fires when you use this action.

For any of the checks, keep track of the value. Your attack roll is the least of all of your checks.

You have disadvantage on any check if the person who did a previous action (or aided) isn't still manning the cannon, or if you did one of the previous actions (or aided).

You can choose to repeat the check if you don't like the value, but if you try it on the same round the roll is at disadvantage.

So max firing speed is 1 per 2 rounds using 2 people with sloppy accuracy. 1 person can fire it 1/4 rounds with sloppy accuracy.

A full crew, meanwhile, is 8 people, up to 4 of them "unskilled".

You can add your Tool(Cannon) proficiency to any of the above checks.

All checks can be aided.

Plagueshot

3d12 necrotic damage attack on all creatures in a 40' cone starting at a point of your choice, facing away from you. Range of 3/4 mile. Gains 1d12 damage for every 2 slot levels used.

Vineshot

Does a grapple check on everyone in a 40' radius sphere and 1d12 piercing damage. Can grapple ships; opposed by pilot check. Range of 1/2 mile. Gains 1d12 damage for every 3 slot levels used.

[False God]

Or a wand that casts any offensive spell, really, you just make it bigger.

When I do "magic cannons" this is essentially what I do. The "wand" casts a specific spell (usually something simple) at a specific spell level (based on materials and some skill checks) and requires a user to fire it. It has a recharge like a breath weapon. They can be built by the player/character if they want something specific, otherwise generic ones that cast something like fireball or lightning bolt are available by specific merchants for VERY high prices.

Usually keeps things pretty simple.

[Throne12]

Just have cannons with the spell catapult infused. Making it a something like a wand. You can do other magic cannons too like fireball, guiding bolt, or chaos bolt.