Portable ship cannon

I'm here for it, if they earn it. I love players having OP bullshit magic, but it's no fun unless they work for it. Changing magic artifacts isn't easy; everything about them is intrinsic to how they work. This is why wizards are useless without their cookbooks detailing every little step, and sorcerers always get weird side effects with their "fuck it, we ball" casting. This is where you tell the players that they're going to need to return to the forge that cast the ring, or find a way to get help from someone high up in the jeweler's guild or something like that. Sure, you could always try to DIY your magic canon, but you're basically doing fantasy electrical engineering with vibes and a screwdriver; ask yourself, what could go wrong?
- That's how I run my table. I am a merciful god, but also a petty god if you reach for the heavens a little too hard. D&D magic already screws with thermodynamics to the point where free energy just exists, so I try to draw a line just short of where anyone figures that part out.
  In the back of my mind, I'm always asking the question: "Why wasn't this loophole exploited in the world already?" That usually prompts a suitable response.
  you’re basically doing fantasy electrical engineering with vibes and a screwdriver; ask yourself, what could go wrong?
  Exactly. In the situation that OP raises, I ask myself: "Does Newton or Gandalf win this argument?"

So does the thing use a portable hole for a magazine or something? Also, does the shrink spell reduce the weight too?
- Enlarge Reduce specifically says the weight changes by 1/8th every time you go down a size category.

I think the party missed another interesting (albeit elaborate) use.
Find a large pointy rock. Shrink it down to the size of a small arrow head. Attach it to an arrow shaft such that the head will slide off the end of the shaft without too much effort (staying embedded in the target). Have the rogue launch it at the big bad, then have the barbarian or monk punch the arrow wound while wearing the ring.
- This is creative. I like this.

I think this ring would be great for handling magic items without being affected by them. Safe way to put something sketchy looking in a sack for later study.
- Doesn't the ring remove the magic though? That seems to be how OP's cannonball gun would work. Otherwise they'd shrink back down after passing through the field.
  
  I had to look this up: you are correct. The RAW on this is pretty clear: It's a suppression field alright, and not a cancellation effect.
  So, it's probably more useful for wielding a cursed weapon or handling magical traps. It would also be great at disguising magic items from detection under pretty specific circumstances. All-in-all, just the thing a thief or rogue would want in their toolbag.

I love how everyone is discussing the physics of a cannonball gun DIY setup in a game where magic can instantly teleport people or turn a person into a huge dragon.
I'm not complaining, I just find it amusing.
- Point is well taken, but D&D magic doesn't take physics off the table, it violates physics within strict limits. Mundane physics still operates. As a DM a good reason I can think of for invoking physics in cases like this is that the player's plan depends as much on physics as it does on magic, and I don't think their cannonball trick would work. The gunpowder imparts the same momentum to the shrunken, diminished-mass cannonball as it would to a regular bullet. When the cannonball's original size and mass are restored, it still has that much momentum - which I imagine will carry it a few feet.
  Fortunately my game group includes a very smart player with a master's degree in physics, who is very quick at computing such things. I would absolutely trust her estimate of how far the cannonball would go.
  
  Inversing this gets really fun: enlarge spell on a tungsten/mithril bearing or toothpick. Pack enough charge to fire it out of an actual piece of siege equipment, hello nearly relativistic projectiles. Ship or battlement mounted rods from God.
  
  I came here to point out exactly this: If you only shrink the ball, without reducing its size, well... you're gonna have problems carrying the ammo.
  As a DM, I think I would let them both shrink and reduce the mass, and wait till they fired the weapon before invoking "conservation of momentum" and declaring that the cannon ball drops to the ground after about a meter.
  
  Yeah, but D&D also has spells and potions of shrink / enlarge that don't make players start drowning in atmosphere their lungs are too small to breathe or collapse under their own weight due to square cube law restrictions, so there's definitely some magical physics at work in there.
  
  Well in that case, the reverse would also be true. So my barbarian can throw a cannonball at someone. How about if the mage readys the shrink spell to target the cannonball just as it leaves my hand? Conservation of energy would dictate that:
  Decisions like these are what makes TTRPGs so fun, and I enjos every minute of these sort ot discussions.
- I see people make comments like this about shows, movies, etc. and I've never understood this line of thinking. I generally expect things to work the same as they do in real life unless it's explicitly explained otherwise. Not sure if I'm the odd man out in thinking that way or what.
  
  you sorta said it but an exception is places like the fae wilds, where you assume physics is only barely present enough to hold your organs together (hopefully)
  
  No, you're right IMO. Just because something is different from our world doesn't mean all logical consistency is off the table. This idea is called versimilitude.

if the ring permanently ends magical effects that enter its area of effect, that's unusual and probably has a bunch of unexpected uses.
It it merely suppresses magical effects in its area, I guess the projectiles would briefly return to full size when in the anti-magic field, and return to small size afterwards? Doesn't seem very effective unless you like point blank someone.
- "Cannonball Punch" sounds like a fun move, though.
- It wouldn't work on a magical lock if you had to hold it up to it all the time
  
  ...what? With the exception of mage hand legerdemain, you're typically very close to locks you're picking. If it made an anti-magic field like 6" from the ring, that would be perfectly fine for picking the lock. Ring is on hand, lockpicks are in hand, magic lock is disabled.

I imagine that the momentum would be conserved. So if the rifle normally shot a 30 gram ball at 300 meters per second, it would shoot a 5 kilogram ball at around 23 meters per second.
The larger size and lower speed of the cannon ball would likely reduce the range.
The larger size of the projectile would spread out the impact causing reduced damage.
The ballistics would be significantly different making it far harder to hit with.
This is how I would do it in my game:
Reduce the damage from 1d12 to 1d10
Change piercing type to bludgeoning
Reduce range from 40/120 to something like 20/60
Add knockback of 5 ft to medium targets or 10 for small
The really neat thing would be shooting non standard rounds that wouldn't be possible from a musket like incendiary or smoke rounds.
- This exact mechanic is present in the Misborn book series. I don't want to give spoilers because I recommend the books so highly, but I love the hard science nature of the way the magic system interacts with physics.
  
  Searching Misborn gives me too many different results. Which one?
- I think damage reduction would be even more than that. The damage a projectile does to a target is directly related to its kinetic energy which is calculated as e = ½mv². So when you increase mass but reduce velocity you also reduce the damage by the square of the difference in velocity (I think). As long as the damage just relies on the physics of the projectile and not magic, that is.
  
  I was assuming that the total energy would be maintained (In this case 1350 joules) and thus the damage should be the same if weren't spread out. It has been 20+ years since I has to do any of that math so I could be wrong about any of that. And since the only paper that was handy happened to be an envelope I guess it was technically back of the envelope math. :)

Wouldn't it dispell the magic before it got to the ring? So your gun just exploded and your ring is now somewhere downrange?
- You'd have to mount it on a wire a bit past the end of the barrel, or custom create a barrel that expands toward the end. Depends on whether dispelling the magic is an instant transformation, or if it "grows" at some rate.
  
  If you can shrink and expand stuff instantly, the thing getting bigger or smaller is the least useful part of that spell. You wouldn't even need gunpowder to launch stuff, put a shrunk cannonball against a wall, expand it, hope the wall holds and it's the cannonball that has to accelerate to light speed to not be in the same place as the wall. Or get a giant explosion, that's more likely.
  
  If it dispels magic before it gets to the ring it's going to be an issue...
- Yeah, if its range is enough to dispel a lock, then it must be at least an inch. So the cannon ball grows while an inch down the barrel.
  
  Meme is still correct though as that's my face while calculating what to change so they don't TPK or something when they try it on the next encounter....

So that is engineering. Is this character an engineer with knowledge of magic, physics and mechanics?
It's fine and easy for a player to think in term of game mechanics. But the actual process is not so goofy. And the character is not the player. The dice decide after that.
- I dunno sounds like the only even vaguely engineering part is glueing a ring to the end of a pistol? If that's considered out of the box clever enough to require a check I can only assume D&D takes place in the systemic lead poisoning dimension.
  
  I would hope they put the ring on a stick so the bullet-size cannonball doesn't enter the antimagic field until it clear the barrel - otherwise it would break the barrel open. But even if they did that I don't think the scheme would work, because when the cannonball's original mass is restored it would have the same momentum the gunpowder gave it when it had the mass of a bullet, which wouldn't carry a cannonball very far.
  
  I think the engineering part kicks in once the cannonball leaves the ring, or maybe around the mass of the shrunken ball. If the cannon ball retains it's mass in it's shrunken size does the gun have enough power to move it? If it does, then the gun is a ship cannon already, just a convenient size. If it doesn't and can only shoot because the balls are as easy to fire as regular shot, then as soon as the ball exits the ring it is a cannonball being moved with the force of a small shot and likely drops to the ground an inch or so past the muzzle.
  
  Many of the things we take for granted as obvious these days were anything but until recently. Take bolt cutters for example. The compound lever that let's them function so well seems like something that would have been around for centuries, but in reality wasn't something that was widely used/understood until the 1890s when they were marketed as a wonder tool.
  On the other hand, this is a game and should be fun regardless of how anachronistic it is at times. At least as long as the witch/duck proportionality is maintained. There has to be at least some realism.

If it is a muzzle loader, no you don't.
- I suppose you'd have to remove the ring while loading in that case?
  
  Good way to lose the ring.

Because I hate fun, I assume that the people in a fantasy world aren't all fools so if there's an application of magic that seems obvious but isn't already happening in the setting, there's a good reason for that.
- There's also the rule of "if you can do it, so can NPCs". Even if your character has thought of something no one in the universe has thought of before, word will get around.
  
  I have it as: "cool idea, i'll let you get away with it once. Start abusing it and all the NPCs start using it." works pretty good.
- This is a good take, but it partly depends on the setting. Specifically, if these magic artifacts are fairly rare and valuable (even something meager like this ring), it's entirely possible that people haven't explored that kind of application of magic. It could also be viable if there are very few inventors/scholars in the setting.
  In any case, the conservation of mass thing another commenter mentioned would make this less viable, so you're right on the money. That being said, laws of physics can be bent for rule-of-cool if that's your table. Personally, if I were DMing it, I'd probably try to find a way to balance between realism and making their research process hilarious and/or dangerous, with the end result being them producing something useful but not gamebreaking (e.g., you can carry and deploy the cannonballs, but the gun doesn't really fire them, but in combination with a method of flight, could still be awesome--or they apply this method with a large boulder and have that to work with instead).
  
  Or, if you want to shut it down, the plan works, but since the magic effect has a range, to ball begins expanding before it makes it to the ring, taking the end of the barrel and the ring with it. If the ring is indestructible the character has to go on a search for the ring at the point of impact every time, it i expands within the ring, meaning you have a canon ball that explodes at the end of your gun with shrapnel, or you have to melt the ball down to retrieve the ring downrange.
- Sounds as if you play as most unimaginative character that does everything in a dullest way possible. What's the point of playing tabletop then? I mean half of the fun of tabletop rpgs is exploring the possibilities and creativity (for me).
  
  The vibe I like is "gritty action movie". Some people prefer something zanier.

Would the momentum stay if the mass increases?
- Lol the ball just falls out the end like a defective Acme gun
- As another poster mentioned, this is likely the reason this isn't already done in the fantasy setting. Either the mass is the same (in which case your flintlock isn't going to launch it terribly far) or the mass changes and it would reduce momentum.
  That being said, it's still a useful way to transport cannonballs, and could still be quite useful. Just not quite a "free" Catapult spell on demand.
  
  If the mass increases after the ball is already moving, then velocity should be conserved and momentum would increase with the mass. That breaks all kinds of physics rules, but this is DnD in a magical universe, there are worse violations going on.
- Momentum could still be conserved if the velocity is unchanged, but it would mean there's now a lot of kickback once it gets big...