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.