Source: Warhammer Fantasy: 4th Edition

Shooting (Skirmishers)
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Skirmishers are difficult to hit with missiles as they are relatively small, individual targets able to take advantage of whatever cover is available. Any enemy shooting at them therefore suffers a -1 to hit penalty. This is, in effect, the -1 penalty normally applied for shooting at characters. A character cannot 'skirmish' and claim a -2 penalty!

It is not possible to shoot through the gaps between skirmishing models with bows or other missile weapons. This enables skirmishers to act as a shield for the units behind them, sheltering them from enemy missile men while they advance.