Lephturn- I noticed nobody actually answered your question. The answer, as far as I can tell, is that they don't. This is based on my personal testing of the B-17 guns with the .target command, and from Hitechs explanation of how buff guns work. Now, don't get me wrong, I used to swear up and down that the guns in bombers were 10 times stronger than their equivelants mounted on the fighters, I am now 99% sure I was wrong and they were right (they being all the people who said it was the same when I was the one squeaking that they were 'porked').
If you are firing from the waist gun at a target at say 3 OC high, where the top turret can also track it, it actually fires parallel to your waist gun. So if you are aiming 30 degrees up (relative to the horizon) to fire at this bandit, the top turret is also firing 30 degrees up from its position (lets say) 10 feet ahead of you and 5 feet above you. This means there are two seperate streams of bullets leaving your bomber- one from position A (waist gun) and one from position B (top turret) that are actually aiming at different spots. You are aiming at position A (bandit) with the waist gun, the top turret is aiming at position B (some point in space 10 feet in front of the bandit and 5 feet above it).
This 'no convergence' has two seemingly opposite effects on lethality. One one hand, it makes buff guns far less lethal, since there is really never more than one gun shooting AT you, although the other 'slaved' guns ARE shooting in your general direction. This means a 1 second burst from a B-17 isn't NEARLY as destructive as a 1 second burst from say, a P-47. On the OTHER hand, it really fills the sky with lead. This has a positive effect on the lethality. Since every gun has a 'cone' that the bullets end up in (because not every bullet flies in exactly the same path), you end up getting hit by bullets from guns that really weren't aimed at you. For instance, currently the most lethal places to attack a B-17 from are directly head on and directly , uhm.. tail on (12 OC level and 6 OC level, from the B-17s perspective). This is because the top, nose (and tail), and ball turrets are all within 10 feet or so of each-other vertically, and when coming from 12 or 6 OC level there is NO horizontal seperation. So if you are in the nose turret shooting at a target coming straight at you from d1.0 out, the top turret (firing parallel to you) is firing at a point perhaps 5 feet above the bandit, and the ball turret is firing at a point perhaps 5 feet below the bandit. At 1,000 yards, the 'cones of fire' for all 3 guns overlap, meaning you get friggin hammered as the fighter coming in. Same story in the tail attack, except the B-17 has even more time to fire at you.
To try to clarify this using the opposite extreme, lets say you attack from 3 OC level. I'm not sure if the B-17s nose and tail turrets can track far enough over to hit a target coming in at exactly 3 OC, but I'll say they can, just because. Anyway, you are in a B-17 cruising along, when I come tearing in on your 3 OC with my 109. You, being the smart man you are, remember which key it is to switch to the right waist gunner (me being stupid, I always forget and get shot down) and start hammering away at me. Since the nose and tail turret are slaved to you (and the flight engineer fell asleep in his top turret, to make the example simpler), they start firing also. You are aiming right at me, but you miss a little high, so I don't die right away. Since there is no convergence on buff guns (because they fire parallel to you), your nose turret swings 90 degrees to the right and opens fire. It is shooting at a point perhaps 50 feet to the left of my plane (from your view), so it doesn't hit jack, even with the cone of dispersion. Your tail turret swings 90 degrees to the left and opens fire at a point about.. I'm guessing here, lets say 20 feet to the right of my plane (again from your view). It has a decent chance to hit me with a few bullets, depending on how far away I am. So, if you are a really good shot with the waist gun, you might shoot me down. Probably not though, since even though all those guns are firing, they aren't all shooting at ME.
I think the problem is that most people come roaring in right up some B-17s ass, and in that case the guns are firing on parallel tracks that are close enough together they may as well all be aimed at you, and get absolutely drilled and have the typical attitude that since that bomber killed them it must be something wrong with the bomber, not them.
Thats how I see it anyway.
I've found that the best way to attack a B-17 is to use the way the guns fire against it. If you come in from 1 or 11 OC level (or slightly high as I prefer), you are basically making it really tough for the turrets other than the manned one to hit you. Plus it is hard for the guy in the turret to hit you because you are rapidly sliding across his view from left to right as you close, while he remains pretty steady in your sights. If you attack in a dive you can make it even tougher since you are not only sliding rightward on his view, you are diving as well.