I had the same initial trouble with the 50 cal birds...didn't seem to be able to do much damage.
All of the above points are outstanding and excellent...but I might suggest another 'theory' if you are somewhat new to the game.
Consider this:
You are flying a plane going anywhere form 150-500 MPH in a fight. Your guns are firing rounds much much faster than you are. You are probably manuevering in multiple dimensions (the X, Y and Z). Your TARGET is doing the same unless you are attacking a set of buffs who are only moving in one (level plane from points A to B in the air).
(X = horizontal component, Y = elevation or altitude component, Z = forward velocity component)
My point? Your gunnery MUST be as stable as possible and you have to concentrate fire on a place for more than a split second to get a kill in most cases. Cannons have the nice feature of being able to kill with one or two rounds on target in most cases (they go BOOM on impact).
I found that once I tried to understand this a bit more, I was effective in using 50 cal platforms.
You have to use them as 'saws' and saw through your opponent's aircraft. This takes some practice and patience. Once you learn to control your own platform and correctly lead your opponent with your controlled platform, you will have better success concentrating fire (around your convergence point), and thus, killing your opponent.
Keep practicing...its WORTH it. I LOVE 50cal planes and fly them nearly exclusively. The corsair and pony are two of my favorite rides. Now I am working on my P38 skills.
That reminds me...stick with one or two planes a t a time, you will learn faster and be able to have a few defacto 'lethal' rides when you need them the most.
And - I use 375 or 400 convergence on all rides...my 'comfort' zone

good luck!!!