Assuming that the cannon did not misfire, then hopefully the cannonball will bounce straight forward and crush any targets in its path.
To determine how far the cannonball bounces, roll the artillery dice again. If the result is a misfire then the cannonball does not bounce – it thuds into the ground and comes to rest. Any model under the spot where the cannonball comes to rest is hit, but they are the only victim!
Assuming you don't roll a misfire, then the cannonball bounces the distance rolled – extend your tape measure a distance equal to the roll of the dice. If the cannonball bounces into impassable terrain that would, in reality, stand in the way of the shot, such as a sheer cliff, it stops immediately.