Pivot the cannon on the spot so that it is pointing in the direction of the model it is going to fire against. Declare which enemy model is going to be the target (remember, the target has to be visible from the machine itself) and then declare how far the cannon is going to shoot, up to its maximum range (eg, 24", 30", 32", etc). Do this without measuring the distance to that target, so try to guess the range as accurately as possible.
Once you have made your guess, roll the artillery dice and, assuming you haven't rolled a misfire, add the score of the dice to the distance that has been declared. The cannonball travels the distance that the player has nominated plus the score of the artillery dice from the cannon itself towards the target, and will either land short, pass straight over, or hit depending on how accurately the player guessed the range and what effect the dice roll has.
Remember the dice will always add at least 2" to an estimate, and can add up to 10', so you should guess a few inches short of the target.
Once it is established where the cannonball hits, place a small coin or other marker directly over the spot. Hopefully, the cannonball will not stop where it hits the ground, but will bounce straight forward and cut a line through any targets in the way. To determine how far the cannonball bounces, roll the artillery dice again and mark the spot where the cannonball comes to land. If the cannonball bounces into impassable terrain that in reality would stand in the way of the shot, such as a sheer cliff (as opposed to a lake), it stops immediately, but other than this, nothing can stop a cannonball!
If you roll a misfire on either rolls of the artillery dice, something has gone wrong, see the rules for misfires given overleaf.
Any models between the points where the ball first strikes the ground and where it eventually comes to land are hit by the bouncing cannonball.
For example, a cannon is fired at a unit of Goblins. The player makes his guess and declares that he is aiming 12 directly towards the middle of the Goblin unit. Having made his guess the player then rolls the artillery dice and scores a 4. which equals 4". This makes a total of 16". He measures 16" towards the Goblins and places a marker where the ball hits. If he has guessed well, this will be just in front of them. The ball now bounces forward the score of the second artillery dice roll. This time an 8 is rolled and the ball bounces 8" straight through the Goblin unit, hitting all the models in the way.
This is completely straightforward in the case of skirmishing units and war machines and their crew, in which case any model touched by the line drawn by the bouncing ball is hit. However, when a cannonball hurtles through a ranked-up unit, only one model per rank can be hit. Of course if the cannonball is coming from the flank of the unit, the unit's files will count as ranks. The diagrams below show how this works.
If a cannonball hits a model that has several parts, like a character riding on a monster or chariot, resolve which part of the model is hit just like shooting with bows. For example, in the case of a cannon hit on a character riding a chariot, roll a dice: on a 1-5 the chariot is hit, and on a 6 the character is hit.