Diagram: The Imperial Guard player fires a mortar, guessing the range to the target (an Ork mob) to be 32". A Scatter dice and a D6 are rolled, and the template is moved 5" in the direction shown by the arrow. As none of the Ork mob are near where it lands, the mortar has missed.
Certain blast weapons launch their shells high up into the air so that they plunge down a distance away, passing over any intervening obstacles en route. Weapons like these fire by guessing the range to the target. They are shown by their maximum range being preceded by a G (Guess), eg G48". When you fire barrage weapons you have to state the target unit and then guess the range to it in inches from the firing unit. Intervening obstacles and terrain do not block the line of fire for these weapons so you don't need a line of sight to the target in order to shoot at it.
Place the Blast marker for the weapon at the distance guessed and then see if it landed on target by rolling the Scatter dice and a D6 together. If you roll a HIT symbol the shot lands at the range guessed. If an arrow symbol is rolled the marker is shifted in the direction indicated by the arrow a number of inches equal to the roll on the D6. Calculate hits on enemy models and remove casualties as for blast weapons (ie from beneath the Blast markers).
If a unit has more than one barrage weapon they will all be fired together in a salvo, with one Blast marker landing for each weapon firing. Guess the range and roll for any potential scatter with the first shot as above, this is the 'ranging shot' for the salvo and all the other shots will land nearby. Once the first marker is in place roll a Scatter dice for each other Blast marker in the salvo. If an arrow is rolled place the marker in the direction indicated so that its edge is touching the edge of the marker placed for the ranging shot. If a hit is rolled the firing player may place the marker so that its edge is touching the edge of any of the Blast markers in the salvo which have already landed.
Diagram: Multiple Barrage (three weapons firing). Three barrage weapons fire together. The 1st round lands as the ranging shot. A Scatter Roll for the 2nd round shifts its marker so its edge touches the edge of the 1st marker (2nd round). A Scatter Roll for the 3rd round shifts its marker so its edge touches the edge of one of the earlier markers (3rd round).