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

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?”

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