@the oncoming storm
First, there was going to be a launching sled, not a pringles can or sabot. I think I characterized the barrel wrong; instead of grooves I meant more like slits with a bolt in them, much like modern automatic weapons. The launching sled would have bolts that would engage the 2 slits, and if the slits were helical, it would make the launch cup spin and therefore the balloon as well. Also, if the spin increases the chance of the balloon popping, then a lower, quicker reload propulsion could be used, such as a powerful spring. This would make a mini underbarrel WBL possible since it would have relatively fast reload and a quick trigger. As a guideline, I'm not as worried about battle practicality than I am about the mechanical concept and if it works.