There is a rear view factor (objects in rear are closer than they appear), but I would be surprised if its average is more than 100 yds. Do some formation flying and I think youll find it not near the issue it was in older games.
Some guns are especially good at reaching out there. 20mm hispanos for one, and also any centerline mounted package. Ive blown up a p51 at 1.0+ with a half second burst in AH2, and commonly connect 800yd shots in the verticle. But then I fly the 38 which has 5 nose guns.
I agree stick movement can be a gunnery factor. Just like in rifle marksmanship, you need to learn to squeeeez the trigger. Not pull, not jerk, but squeeze. Also, you might want to check out the other thread on AH2 gunnery "How do you guys do it?"