How does choosing attacks work?

If Rhymer with APT is now attacking against a R2 ship and R3 Biggs, does he have to shoot Biggs with his primary, or can he fire on the R2 ship with the APT?

He would have to use his primary at R3

You have to declare your target before you declare your attack type, so you'd have to attack Biggs before choosing to use a missile.

Gotcha. Awesome.

FAQ, Page 6:

If the attacker has more than one weapon available, it must use a weapon that can target Biggs Darklighter, if possible, instead of any other ship at Range 1 of Biggs Darklighter