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

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.

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.

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.

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?"

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

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