Look... this is how it was explained to me, and some stuff that I did to test it.
Buff guns do NOT 'converge' at a particular spot, EVER. What they do is fire perpendicularly to wherever you, in whatever gun you are using, are firing.
So for instance-
I see a really stupid C202 that wants to try to sit on my 6 oc and pepper me with the lil bitty guns he has. Lets say he is actually exactly behind me. Since there are some 'issues' with the firing arcs on B17s, an enemy fighter attacking from directly behind will be fired on by the ball turret, tail turret, and top turret (as opposed to real life, where if you were EXACTLY behind the B17 only the tail turret had enough traverse to hit you, the ball turret couldnt go that high and the top turret couldnt go that low, plus it would shoot the living hell out of the vertical stabilizer if it was shooting in that direction).
Ok, here is where the really fun stuff starts. A b17 is X feet tall from the ball turret to the top turret (lets say X=10 for my purposes). So what you have is a stream of fire going from the tail turret at Y height (let Y=0 for my purposes)- plus a stream of fire from the top turret at Y+5 feet, and a stream of fire from the ball turret at Y-5 feet. Obviously, not all of the bullets fire exactly straight. You will get some going to the left and right, and some going up and down (and really adventurous ones going up and left, or down and right, etc.). This will obviously happen to ALL three streams of fire coming at the hapless C202. So what you get is that the rounds that go reasonably straight from the tail turret slam into the C202, plus the rounds that wobbled down a bit when they were coming out of the top turret, PLUS the rounds that wobbled up a tad when coming out of the ball turret.
It takes roughly 25 .50 caliber rounds to knock a Fw-190A8 out of the sky with the engine gone and the wing taking a seperate path to the ground. I don't know how many rounds it takes for the C202, I've never bothered to test it.
How long does it take for 6 .50 caliber machine guns to fire enough rounds that 25 of them hit? My gunnery is usually about 10% or so, but if there was a guy just hanging motionless in my gunsight (and that is exactly what you look like to the buff if you are sitting on his 6) I'd guess it may jump as high as 20 or 25%. So, the ROF for a .50 caliber M2 machinegun is (I gotta go look this up) 800-900 rpm (according to Americas Hundred Thousand).
Math has never been a strong suit of mine, so this may take a while.
1 .50 caliber MG fires 850 rpm. Divided by 60 gives me the rounds per second. Roughly 14 rounds per second. I need 25 rounds to hit to kill the plane. If my gunnery is my normal sucky 10%, I'll need to fire 250 rounds. Dividing 250 rounds by 14 (for ROF) gives me close to 18 seconds. There are 2 .50s in the tail turret, so if that was the only one firing I'd need to shoot for about 9 seconds. Add in the top and tail turrets, that gives me 6 guns total. 18 seconds divded by 6 guns gives me 3 seconds.
So if I was my normal crap shot, it'd only take me 3 seconds to blow away a 190A8 that was sitting on my tail.
If I was having a good shooting day, and was hitting with 20%, it'd take me a second and a half. I don't think the game can even play the sound of 25 rounds hitting you in a second and a half, hence the 'That @#%$#^@&% buff shot me down with ONE #@$%^ ping from 800 @#%#$^^ yards!!! They are SOOO $%^$#@& porked!'
Anyway, that is about as well as I can explain it. Hopefully someone more eloquent than I will step up to the plate if people still disagree.