make manned guns count as kills. there is no valid argument against it when you have somthing like the jeep that counts as a GV kill.
and go the full mile make the m3 and sdk able to tow a field gun like a drone.
the sdk/m3 can then park a specified distance away before the field gun dies from range like a drone.
you get 3 kills for wiping out a formation of bombers manned by one guy. there should be no issue with getting two kills for wiping out an sdk/m3/truck and field gun and getting two kills.
and if you really want to have some fun make the field gun have no icon since it is unmovable when not connected to the towing vehicle and watch how popular it becomes in spite of it being a soft target.