The gunners are generally "funky". For example, if you try a QMB mission against some B17s you can see that half the gunners are shooting in the wrong direction and the other half will be sniping you.
According to what i've seen when testing ways to attack bombers, these are my observations. The gunners are super accurate,
as long as they are already facing towards you when you are coming in. That's why the tail gunners are usually deadly. Coming in from the sides in slashing attacks with an early model FW190 (no Mk108s) you can wipe out four B17s with veteran or ace gunners. I have done it in a Fw190A3 that has the MGFF outboard cannons.
The trick is to fly in a manner that even when you fly straight, your course will be at such an angle to the bomber's course that will force the gunners to change their aim and do so rapidly. Coming in from his 4 o clock for example with a slight lead and firing you'll see that you rake his entire fuselage, passing from nose to tail as you exit the firing pass. From the gunner's point of view this looks like a fighter moving at 500kmh sideways from nose to tail.
The AI gunners have a problem turning their turrets fast enough and you can see it if you fly in a B17 (in a modded install), switch to the top turret position and watch how the gunner works when he's on autopilot.
On the other hand, they start firing whenever an enemy is at a certain distance according to their level (rookie, veteran or ace), no matter if they have the fighter in their gunsights or not. Try a QMB with sideways slashing attacks and watch the track afterwards. Suppose you're attacking from their 4 o'clock and exiting to their 8 o'clock. They start shooting and keep shooting where you were (their 4 oc) and can't really turn their guns around fast enough to keep tracking you. If you watch such a track you'll see loads of tracer fire going in the wrong direction.
I think this is done as a way to offset their accuracy when they do happen to have you in their general field of view. So in summary, they seem like they don't even know you're there until you close to a certain distance from them. I think they have a certain detection range and that detection range is the same as their reaction/open fire range. If you man the guns yourself, you'll know where the enemy comes from and turn the turret around well in advance. The AI on the other hand will suddenly react to enemy presence at 1.2km or so (depending on the AI level), even if you're visible from way out.
At that point, if they have their turret pointed the right way they can shoot at you and they are accurate. If they happen to have their guns turned somewhere else, you're pretty much in the clear as they won't be able to turn them around fast enough, even though they'll be shooting wildly in the air. Try sitting in a turret yourself and compare how fast you can track an incoming fighter in comparison to how slowly they AI moves.
I hope this will give you some hints on how to attack bombers. For example, in my favorite method i attack in sideways slashing attacks. I have even crossed the dead six of a B17 formation in an online server this way and survived while chopping off their rudders in low deflection passes because i was in a hurry to get them. The proper method involves much higher deflection and even less chance of getting hit.
That's all because of what i described above. When you make your first slashing attack some gunners will shoot at you and some will be turned the wrong way,
but all of them will try to turn their guns on you. So, when you come around for the next pass from the opposite direction, their guns will be facing the way of your previous pass and they won't be able to turn them to you in time!
