Some pieces of ordnance are used for laying down a barrage in the same way as mortars (though on a much larger scale). With these pieces you need to guess the range and roll for scatter just like other barrage weapons.
Place the Blast marker for the ordnance at the distance guessed and then see if it landed on target by rolling a Scatter dice and a D6 together. If you roll a HIT on the Scatter dice the shot lands at the range guessed. If an arrow is rolled the marker is shifted in the direction indicated by the arrow a number of inches equal to the roll on the D6.
Ordnance barrages are also considerably more terrifying than other incoming fire — see Ordnance Barrage Pinning.