This works as a normal volley gun (see page 15 of the Empire book), with the following exceptions.

Hits from the gun are:
Strength 3, -1 save at long range;
Strength 4, -2 save at short range.

If you roll a Misfire on any Artillery dice, roll a dice on the Helblaster Misfire chart and one dice on the Steam Tank Malfunction chart. Apply all results immediately. Note that if you get a 'Destroyed!' result on the Helblaster chart, it applies only to the gun itself, not to the entire Tank.