DnD Memes

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.

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.

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.

This exact mechanic is present in the Mistborn 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.

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.

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.

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?"

I suppose you'd have to remove the ring while loading in that case?

This is creative. I like this.

Enlarge Reduce specifically says the weight changes by 1/8th every time you go down a size category.

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.

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.

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).

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.

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.