A couple of tips that may help. First, you are probably not leading enough on crossing shots, that's the trouble most people have. To hit an aircraft at even 200mph you might have to lead almost a whole screens worth (depends on range). The gunsight itself is basically useless for deflection shooting.
The thing to do is, go offline to a field with drones and shoot at them a bit from an Ostie. Record it when you are doing it and then play back that video and watch where your rounds are going (you can jump into the aircraft by clicking on the names). After you do this a couple of times you will get a feel for how much to lead an aircraft. I bet it's WAY more than you thought. And remember, that's on a simple scenario where the plane is only doing 200mph at 3,000ft. Another thing you can do it turn on the lead-computing-gunsight option (in setup) and then use that to get a feel for the aim. I seem to remember that it only really shows about half the amount of lead you really need on the Ostie.
THe problem with getting killed by manned field ack is usually because you wait too long to shoot. Remember that when strafing ack, it's not moving, so shoot early. Guns have a range of D1.2 or so when you are just strafing the ground, so fire WAY out and let the rounds sprinkle around the ack and kill it. If you are flying directly at the ack then by D800 someone is going to nail you. You need not fly directly at the ack either, fly slightly offline and then turn into the ack and fire, never flying straight at it. That usually throws off most manned gunners. The auto-ack is simply luck though, it auto-compensates and lots of speed doesn't help (some think it is actually worse to have lots of speed).
Hope that helps. Ostie/Field Ack gunning is mostly a feel thing that just takes some practice and I don't know that anyone can reliably hit things that aren't very close or at low deflection angles.
