Bolt throwers are fired in the shooting phase along with other missile weapons. First select a target unit following the normal rules for shooting, drawing line of sight from the machine itself. Now pivot the bolt thrower on the spot so that it points towards its intended target. To determine whether the bolt is on target, roll to hit using the crew's BS in the same way as bow shots, crossbows, and other missile weapons. The usual modifiers apply, except no penalty is imposed for turning the machine, as it is designed to be used in this way. See thesection for details.
If the shot misses, the bolt hits the ground or sails into the air and comes down harmlessly somewhere else. If a hit is scored, work out damage as described below.
If the target is a unit of five or more ranked-up models, the bolt will always strike a regular trooper in the unit's first rank (as opposed to a champion or character in the unit) if the bolt thrower is in the unit's front arc, or the last rank if the machine is in the unit's rear arc. If the bolt thrower is in the flank of the target unit, the unit's files will count as ranks. The diagrams below show how this works.
If the rank hit by the bolt is made entirely of characters (including champions) or if the total of rank-and-file models in the unit is less than five, it will be necessary to randomise which model in the rank is hit.