From what I see, ack targets the nearest plane.  The moment another enemy plane comes into range, ack will instantly start shooting at it.  I've actually seen ack fire 2 shots at a plane that flies by, only for the gun to instantly do a 180 and the next 2 shots in that 4 shot salvo, to shoot at me.  The instantaneous re-targeting thing is a bit "off", to put it mildly.
A crude picture of what I mean is below: 

 <1--  <2--  (gun)  --3>  --4>  

If updating the ack guns, give them realistic traversal times as well?