These weapons are like Barrage weapons, but even more destructive. They are also cumbersome and slow to reload — only war engines are big enough to maintain a constant feed of ammunition to such a weapon during a battle.
When you make a Heavy Barrage shot, proceed exactly as for a Barrage shot but with these differences:
The Firepower value is equal to twice the number of units that are under the Barrage template.
If the weapon is on a War Engine unit then you may continue to move and shoot with this unit each turn as normal. Otherwise, you may not move or shoot again with the unit until it has had a full turn to reload. Turn the model around, mark it with a token or make a note so that you don’t forget that it must reload before you can use it again.