Originally Posted by Engines
an air to ground gun is normally depressed below the line of flight
Not quite. An air-to-ground installation may be depressed from the longitudinal fuselage datum and an air-to-air gun level or above. Traditionally. 'Line of flight' (or velocity vector) constantly changes with airspeed and loading.
In reality and as you explain very well, on a reasonably modern airframe the middle of the road compromise works perfectly well for both roles.