We worked out a way to adapt our potato cannon to fire snowballs. Do not, however, under any circumstances fire your snow cannon at anyone. I'm guessing a hard packed projectile made of snow is going to be quite as devastating as a potato would be. In fact, I know it is because we shot a hole in the shed with ours.
Potato cannon instructions here: http://baddadgreatkid.blogspot.co.uk/2011/03/how-to-build-your-own-potato-cannon.html