More lag -> more distance / angle difference.
Remember that lag between you and enemy might be even 2 seconds (woah..)
Some of guys unfortunately flies with even 1200ms connections (which is 1.2 seconds...!)
If you have 300ms lag to server, and enemy has 1 seconds, its 1.3 seconds of lag between you two.
If you're flying straight when enemy comes fast to your 6, you might see him 1k away when in his FE, hes 300 yards or so. (depending on speed)
When you shoot with 12.7mm guns, you have to shoot same area, not around the target.
I have downed B-17 with pair of 13mm in 109G6 or G10..
Only thing I did, I sprayed only it's right wing. (remember that you'll hit either tip or middle wing, not both)
Hitting wing root might be most effective.