As the older hands will tell you my shooting sucks.
I have worked on this for a while.
there is info posted in the vault - the RAF gunnery guide
http://www.dangerdogz.com/K9Vault/p13_sectionid/5/p13_fileid/241
here are some other training missions I use
http://www.dangerdogz.com/K9Vault/p13_sectionid/5/p13_fileid/212
there is also darts video and over at simhq aircraft library there are articles by andy bush
What professer asays about ping is key - a low ping will improve you a lot , but learn to lead that extra 1/2 ring online
Here is what helps me
1) Get close - 300m max , 200m optimum
2) set your convergence low - 200m
3) If the plane in front is turning, dont pull on the stick to follow, you will always fire behind it. Reposition your plane to get a lower angle off then fire.
4) have patience. the guy has made a mistaske allowing you to get in a firing position, dont blow your position trying desperatly for a shot. Fly smart and keep you r E up and the gun position will come again. Fly stupid and you will , stall/ overshoot/ bleed E and suddenly you are on the defensive and even madder with yourself
5) record tracks and view from externals and from the enemy plane and see how far your bullets miss.
6) good suggestion about using the P-51 gyro gunsight , this really shows you how much lead you need. But remember this is for .50 cals. For 20mm you will need to lead more and for 30mm more still.
hope this helps
S