Treat as a stone thrower in the rulebook with the following exceptions. You may only guess between 12" and 48" for range. Use the cannon misfire table. In addition to normal damage, roll 2D6 once you have worked out where the shell lands. Any troops within this distance in inches from the centre of the template move at half rate in their next turn and may not shoot missile weapons. War machines caught in the area may only fire on a 4+.