When do you choose the target of your BS attack? For example: you have a trooper (A) and its crazykoalas. The first short skill is move and one of the koalas is in LoF of an enemy(B). This enemy declares ARO:BS attack The secomd short skill is move again and A moves into LoF of B. In the resolution step: can B shoot at trooper A or only at the koala? What about a fireteam instead of koalas?
You choose at point of Declaration, so the Reactive Trooper has to either shoot at the Koala, Dodge, or forfeit their ARO. Same works for Fireteams, you can only target models that are visible at the point you are Declaring the Attack.
For reference, it's under BS Attack, Effect 3 http://wiki.infinitythegame.com/en/BS_Attack If the attacker has more than one target and a BS Weapon, Special Skill or piece of Equipment with a Burst value higher than 1, he must distribute his attacks as part of the BS Attack declaration. And again under Burst for BS Attack Declarations http://wiki.infinitythegame.com/en/BS_Attack_Declaration#Burst During the declaration of a BS Attack, the player can distribute a number of shots equal to the Burst value among one or more targets, as long as they are valid (within LoF, if needed).
It's also in the big red box on the OES page. http://wiki.infinitythegame.com/en/Order_Expenditure_Sequence IMPORTANT! All details and choices related to the execution of a Short Skill, Short Movement Skill, or Entire Order Skill must be specified when it is declared. For instance, if you declare a movement, specify the entire route; if you declare a BS Attack, specify which Weapon will be used, who the targets are, how the Burst is divided, etc. Emphasis mine.
