I feel like the "monkeys paw" way to interpret this if it's considered too powerful, is to say that, when the cannonballs grow back to normal size, they keep the same momentum they had when small, and since their mass has increased, that must mean their velocity drops proportionally and they just kinda fall to the ground after leaving the barrel

I usually ask the players something like "if it was this easy, why doesn't everyone else do it?". Sometimes they'll dismantle their own ideas given the prompt.

Or, sometimes I'll say "are you sure you want NPCs to use this tactic?"

The first time the player fires the gun, the antimagic field affecets the cannonball just as it is approaching the ring at the end of the barrel, expanding to full size in the barrels end instantly, and pulverising the firearm and sending the ring off as a prjectile to an unknown destination

Unless that flint lock is a breach loader congratulations on your pipe bomb if you can even get the shot in.

There are several ways to nerf this, if considered too powerful. But I like to reward my players for creative ideas like that. And it's not really powerful either. You shoot non-magical cannonballs. That's OK.

Sure, you got canon ball moving with all the kinetic energy of a musket ball.

I mean, wouldn't the obvious answer without being a tyrant be, to just adjust the followup encounters to still be a challenge? At least in creative ways so as not to just end up with a, "no you" feeling to it or glass cannon vs glass cannon?

I'd prefer to play with a creative DM that would laugh and accept it than one that'll nerf me for being more creative.

DM: *snaps fingers* Done. Calm down, Tom. I'm not going to punish you for being creative. You can get creative as much as you like. Hell, thats the point. ...BUT, just remember, you will never, and I mean NEVER, win. There is no magic combo that lets you 'win' an RPG. You keep going or you don't. There is no 'win' state. For now, they fire the test shot and demolish the tree at which they fired it, plus some others in the ricochet path. They both get an adrenaline rush as they realise how dangerous this could be. Now, what do the rest of the group do with their day?

When a player tries to use physics at my game I ask them to explain how diagonal movement isn't faster than moving in a straight line.

DM: Oh no. You ruined my campaign!

Another player: Wouldn't the canonballs just shrink again once they leave the antimagic field? It wasn't permanently dispelling the magic locks.

DM: Shhh!

Question: if a cannonball is shrunk down to musket calibre, shot, and upon leaving the barrel grown back to cannonball size,

A) Does mass get conserved in the shrinking?

B) What gets conserved upon the transformation back to cannonball size?
I) Velocity
II) Momentum
III) Energy

As someone who's inclined to take this nerd snipe, I kinda want to know.

This is pretty stupid. There’s no easy way to permanently shrink hundreds of cannon balls; so this scenario doesnt make sense.