This works as a normal mortar (see page 13 of the Empire book), except that the Steam Tank's mortar has a range of 6"-24".

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

In addition, the Tank is equipped with two swivel guns. When the Tank is not engaged in close combat, the crew can decide not to fire the mortar and fire the two swivel guns instead (this does not cost any Steam Points). Treat the guns as handguns with a Strength of 6 that always count as stationary, even if the Tank has moved. They can be fired at targets in the Tank's flank and front arcs. Remember that the crew has a Ballistic Skill of 4.