I think it’s pretty clear how AA guns work from reading their unit profile in the Rulebook. However, I agree that the defense value listed could be a bit misleading. On the other hand, this value has been in place since Revised (we playtesters are so awful that we can actually go back in time and screw things up in the past :roll:), and to my knowledge this is the first time that this question has ever been asked. Maybe it would be better if the defense value value were listed as “Special”.
As for ICs, they’re defense is “NA” because they never fire under any circumstances, which is not true of AA guns.