I found this playing SC FA and I'm not sure is also at FAF, but newertheless. So, can t2 seraphim gunfigters attack air with same cannons as ground? And does it make them best air unit?) At least for cheesing strategies.
AA Seraphim gunship?
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I'm confused what you mean. Nothas (the Seraphim t2 fighter/bomber) do have decent AA, but do not use the same weapon they use against ground-units. They have a separate AA.
Voltoos (I hope that was their name? The Sera T2 Gunship) can only target transports and other gunships. They do use their groundweapon against them, since they have no separate AA.
Inactive.
You can see for youself that the Vulthoo doesn't have any separate AA weapon.
The db doesn't clarify this, but if you click on the unit blueprint (XSA0203) you can see TargetRestrictDisallow = "HIGHALTAIR, UNTARGETABLE", which is what Sladow-Noob said.
You might be misremembering as gunships can shoot other gunships and I think transports. But not aircraft.
Thank you, I understood. Next my topic will be about kybran arty, aeon gunship and other units which can be cheese 
@bbi Cybran artillery are SO damn bad compared to the other factions, EMP be damned!
Aeon (t2?) gunships are great for their AOE, but can be comprehensively ccountered using anti-air.
Aeon T3 gunships have to use range and kite opponents, which is stupidly hard given the speed of typical air units. For the most part they try to do what ASFs do, but much worse.
(Someone correct me if I'm failing to understand stuff here!)