In WH the rules are simpler. If the firing model/s can see ANYONE in the unit, they're eligible to kill every last one of them.
This is fair because to charge, only one model in the unit needs to have LOS to the unit they're charging.
That said, this is assessed when you check LOS for each given firing unit. So if you have a skirmishing unit with one model poking around the corner of a building, and your opponent shoots at them with one unit and kills a model, you could select that visible model. Which might prevent your opponent's next shooting unit from having a target.
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