This is a good take, but it partly depends on the setting. Specifically, if these magic artifacts are fairly rare and valuable (even something meager like this ring), it’s entirely possible that people haven’t explored that kind of application of magic. It could also be viable if there are very few inventors/scholars in the setting.

In any case, the conservation of mass thing another commenter mentioned would make this less viable, so you’re right on the money. That being said, laws of physics can be bent for rule-of-cool if that’s your table. Personally, if I were DMing it, I’d probably try to find a way to balance between realism and making their research process hilarious and/or dangerous, with the end result being them producing something useful but not gamebreaking (e.g., you can carry and deploy the cannonballs, but the gun doesn’t really fire them, but in combination with a method of flight, could still be awesome–or they apply this method with a large boulder and have that to work with instead).