Choose a Target

Front rank models can always shoot, and second rank models can too -- they're assumed to aim over their crouching or stooping comrades ahead of them, using that model's own line of sight and forward arc for every shooting purpose (including Stand & Shoot reactions). In short, units can fire two ranks deep.

Models any further back than the second rank are assumed to have their line of sight blocked entirely and can't normally shoot, no matter the target's Unit Strength.

Every missile weapon has a maximum range -- see the Weapons & Armour chapter for a full summary. A model can normally only shoot at a target within that range, with one exception: a unit making a Stand & Shoot reaction is assumed to have the charger already within range by the time it fires.

Every model in a unit has to shoot at the same target -- but depending on where the shooters and the target are standing, some models in the firing unit may be able to see or reach the target while others can't (out of arc, blocked by terrain, or simply out of range). You're free to shoot anyway; only the models that actually have the target in arc and range will fire, while the rest hold their shot (whether out of discipline or simple confusion, the game result is the same -- they just don't shoot).

Units engaged in close combat normally can't be shot at -- there's too much risk of hitting a friend in a swirling melee that only looks like two static blocks of troops.

The one exception: a unit within 4" of an unengaged arc of an enemy unit in combat may attempt to fire at it, provided the target has Unit Strength 5 or more. At that range they can pick out enemy targets well enough to risk the shot -- but for every natural 1 rolled to hit, that shot goes astray and hits the friendly unit instead.

Template weapons can never target an enemy engaged in close combat, even within 4" -- though they can still hit a nearby combat by accident (never on purpose) if positioned near one.