Shovel Knight found primarily in the Flying Machine. Cannonballs are shot out of tubes at regular intervals and can be Shovel Dropped to bounce across obstacles. If Shovel Knight is hit with a cannonball he will take damage. Cannonballs come in two sizes, but both behave identically.
In Specter of Torment however, these cannonballs can be cut into two halves by Specter Knight. He can even perform dash slashes with them.