The FA and ADA split off from each other in the 1950s or 1960s because of different missions and weapons systems. If I recall correctly the Traversing and Elevating Mechanism for the old M2 Browning .50-caliber Machine Gun, originally intended to be mainly an air defense weapon, is graduated in mils, not degrees (6400 mils = 360 degrees), which shows the old Field Artillery connection between the two branches.