By a kilometer out, you mean 1K as per the plane icon? If so, that's 0.5K under the bullets' maximum reach. MG bullets need to hit the gun itself to score damage, while cannons have enough splash damage to take out an AAA gun with a few hits in the white disk around the gun.
Ideally, you shoot between 1.5 and 1K out, so as to spend the least time pointed at the gun (so that it doesnt hit you), and in one direction (so the other guns have to keep adjusting their aim).
Rockets will barely have any drop when you come in on the ground target fast enough (say above ~350mph), and with enough vertical angle.
Killing off individual bomber gunners is pretty good, depending from how far.. you should be able to do that soon enough too, although some planes are much better gun platforms than others, while others such as the 109K4 or Yak9T require you to land a relatively slow firing gun's shells right on target from pretty close, when the ballistics are slow and heavy enough.. which is one of the toughest things to master in the game.