Since your enemies can unload ships into a combat zone before combat begins, you may optionally commit guns to attack those cargo ships as they unload.  Guns committed in such a fashion fire only if enemy cargo is unloaded, and will sit idle if no cargo is unloaded.  Thus, you must weigh the tactical consequences of holding some of your guns in reserve for this purpose.  
 
Your battle computers will automatically target the individual cargo ships as they unload according to the same criteria described in the Fire Guns phase.  Note that if you commit guns to attack cargo, those guns will fire at any foreign ships that are unloaded, regardless of owner, including allies.  Your  guns will fire at the unloaded cargo ships even if those ships do not attack you.  (Consider it a case of an itcht trigger finger.)

The following ships are not considered unloaded cargo and thus cannot be targeted as such:

• missiles that are unloaded and then fired in the same turn.
• devices that are unloaded and then deployed in the same turn.
• cargo ships that are unloaded and then immediately loaded onto another carrier ship during the Logistics phase.