I mean technically this really shouldn't work with the way the field is on the ring (see bethorian's post below) but I would say given the speed fired through that the enlargement occurs as it passes through. The ring itself wouldn't be destroyed regardless as it is a magical item. Probably would work better with a dispel magic item attached somehow.
Based on how an anti magic field works
(An emanation in every direction) the field would unshrink the cannonball before it made it to the end of the barrel causing the musket end to explode, then because an antimagic field is temporary magic negation, not dispelling, it would return to the size of a musketball.
(An emanation in every direction) the field would unshrink the cannonball before it made it to the end of the barrel causing the musket end to explode, then because an antimagic field is temporary magic negation, not dispelling, it would return to the size of a musketball.