Flame Cannons shoot using the Fire Thrower rules in the Warhammer rulebook with the addition of the Multiple Wounds (D3) special rule. However, a Dwarf player can also pump extra steam to fire a supercharged flame shot with the following profile and special rules:

To fire the supercharged flame shot, nominate a point within the war machine’s line of sight and within 12". This does not have to be an enemy model, but can be a point on the ground if you wish. Place a small counter (a coin will do) in the correct position as a reminder. Using your tape measure, extend a 'shot' line from the Flame Cannon’s barrel all the way to your target point. Roll the artillery dice and extend the line away from the Flame Cannon the number of inches shown – this is where the teardrop-shaped template is placed (the narrow end closest to the Flame Cannon). After this, work out hits exactly as per normal fire thrower rules. Note that you are not allowed to make a Flame Cannon shoot in such a way that it has a chance of hitting a friendly unit or an enemy unit that is engaged with friendly units. If you roll a misfire result while shooting a supercharged flame shot, you suffer a -1 penalty on the Black Powder Misfire Chart.