I'd say there are a few exceptions to not gunning past 600 yrds. #1 is a bogey thats flying at your altitude directly in your "6". Once you learn the drop its fairly easy scoring hits at 1,000 yrds. Ive dropped quite a few fighters from that range. The belly guns are difficult to hit long from however. The top turret is easier tho the angle might be hard.
Another exception is a heavy Jabo thats loaded with guns and cannon. I dont let planes like the 110 or Mossie get within 600 of my bombers if I can help it.
Another exception is i just might shoot long to rattle an enemy, or, see If I can rattle them.
Another is with the B-26 which has 1,600 rounds in the tailgun. Ive never ran out of ammo with it and always begin shooting at fighters in my 6 at 1,000 yrds.
Lastly much depends on where your at in your mission. If your deep behind the lines, or inbound to your target, you want to be a little stingier with your ammo. Conversely if your close to home, with friendly ahead of you, you can afford to let loose a little.
One thing is I never take a shot that I cannot think I can make. The 0.50 cal is fully capable to kill an enemy at 1,000.