This works as a normal cannon, with the following exceptions:

If you roll a Misfire on the first Artillery dice, the cannon does not fire and you must roll on the Steam Tank Malfunction chart. Apply any result immediately. In addition to the effects listed, if you roll a 1 on the chart the cannon is destroyed, and if you roll a 2-3 on the chart, the cannon cannot shoot in the following turn.

The Steam Tank's main guns cannot fire grapeshot, but any enemy unit in base contact with the front of the steam Tank when the main cannon is fired (ie, standing in front of the muzzle!) suffers D6 Strength 3 hits with a -3 Armour save modifier from the overheated steam released.