Grape or case shot might be a better pick; a shotgun effect to maximize the chance of hitting.
The problem is the face that you're shooting these over your own territory. Don't really want shot raining down over the populace. Ideally, you'd just put holes in the balloon so it descends gradually, and you'd have to chance to warn those below that it might be coming down. That'd give you the option to bring it down earlier. Grapeshot using hard-frozen ice chunks come to mind, but IIRC, the Mythbusters showed it wouldn't survive being fired from a gun. May not melt on the way down, either.
Ron Wanttaja