If the target is a unit of five or more ranked-up models, the bolt will always strike a regular trooper (as opposed to a character in the unit) in the unit's first rank if the bolt thrower is in the unit's front arc, or the last rank if the bolt thrower is in the unit's rear arc. If the bolt thrower is in the target unit's flank, the target's files count as ranks for the purposes of resolving the shot.
The sheer mass of a bolt thrower shot means that it can hurtle through several ranks of troops, piercing each warrior in turn. If the shot hits, resolve damage against the target using the bolt thrower's full Strength of 6. If this model is slain, the bolt then hits one of the warriors in the rank behind, although it will have been robbed of a little force – this next warrior is hit with -1 Strength (normally Strength 5). If the second trooper is slain, a model in the next rank is hit at -2 Strength (normally Strength 4) and so on, until either the unit runs out of ranks or the bolt fails to slay a target (remember that all hits, at any Strength, inflict multiple wounds).