I can't remember were I read this, but my understanding in AH2 was that...
2 x .303 = 1 x .50 cal
2 x .50 = 1 x 20mm
(approximately)
.303s, from my observations are pretty much useless beyond 300 yards.
.50s tend to do very well out beyond even 400 yards and have about the flatest trajectory for range in the game.
20mm have great all around hitting ability, but drop quicker and have a slower rate of fire the the .50s. However, if you know what you are doing and have a plane with 2 x 20 mm or great you can hit at a pretty long range (assuming convergence is set well).
I used to straffe ACK emplacements (when AH2 first became the only arena) and take them out with about 70 to 100 rounds if I put the rounds in the center of the gun pad (less if I was more accurate). I did this without having to get all the way in on the gun (like, pulling up at the absolute last second). Now, I have made two or three passes at ACK and is still be in good form.
Against enemy AC I am really having to get in tight anymore with the .50s to do any good...unless, of course they are flying straight and level and I am about 400 to 500 yards off.
To me, from what I have seen, cannon armed planes rule anymore. If you aren't flying a cannon mount, you are just a bunch of ammo waiting to get shot. Doesn't mean I don't get kills with the MGs, but it does mean a lot more work. I used to love to fly the Jug (who doesn't love jugs

?), but I shy away from it now because even those 8 brownings are quiet as deadly as the seem to used to be.
Oh well, just my ramblings...