So APIT does more damage I take it? Does it normally have tracer and ball then?
APIT just does slightly different damage. It combines some of the armor-piercing outer core of an AP (armor-piercing) round, the tracer aspects of Tracer and some of the incendiary action of an Incendiary.
It's good for taking out lightly armored targets like fighter aircraft and light armored vehicles. I also believe that it has somewhat closer ballistic performance to ball (regular), AP or API (Armor Piercing Incendiary) than regular tracer, and obviously has better performance than tracer if it hits its target.
The USAAF used a lot of APIT and API during ground-attack missions in the ETO and MTO towards the end of the war, since it worked well against unarmored and lightly armored vehicles. It also worked fairly well against fighter aircraft, which is mostly what the U.S. encountered over Germany. By contrast, the Axis encountered more bombers, so they needed heavier-hitting guns like 20mm or 30mm, which fired shells which carried a high explosive charge.
For heavier prey, like tanks, I believe that straight AP rounds were preferred, with some tracer or APIT mixed in.
For unarmored targets, ball (i.e., regular) ammo was fine. I believe that in the Pacific, U.S. aircraft were just armed with a combination of ball, incendiary and tracer or APIT, which worked better to set unarmored Japanese planes alight.
Discussion of WW2 era U.S. fighter guns here:
http://www.thehighroad.org/archive/index.php/t-472212.htmlProbably the best, and most commonly referenced web page on WW2 era aerial guns here:
http://www.quarry.nildram.co.uk/WW2guneffect.htm