In addition to Vinkman's good points, my general rule of thumb with 30mm is to get in C L O S E, and give it a lot of lead until you get used to it. I still shoot behind people all the time particularly with 30mm. I don't like to fire outside icon 200 and I prefer to be right on top of them. The dispersion on the 30mm is extreme. You can be dead on a fighter at icon 400 and shoot all around his plane without touching it, particularly in a tail chase. Bombers slightly less so.
Wiley.