While the bomber drones are convergent at 500 yards, the guns themselves will hit at distances past 1k in many cases. This is because the bombers fly at super fast top speeds, which is totally unhistorical. This means, basically, that they can fire further away at targets behind them because they (the targets) are flying towards the bullets, closing the distance that much faster while the bullet is in flight.
In the 1 second it might take a bullet to travel, a plane flying at 350mph can cover half a mile.
Gunnery is also dependent on the atmospheric conditions. If your bomber is on the deck the air is much denser, and bullets slow down faster. That means return fire is probably around 800-1000 yards lethal. If your bomber is at 25k+, however, it's quite possible to get hits out at 1.2, dare I say 1.5k (perhaps not QUITE that far).
There are plenty of factors to take into consideration. In the case of the lancaster I would say there are 2 weak zones:
1) Directly top-down. The cockpit glass is unarmored. A good solid sustained burst can kill all 3 planes as they pop into lead, if you instantly kill the pilot 3x in a row. Usually 50cal planes are best for this.
2) directly BELOW. It has no lower guns. However the tail gun can shoot pretty low, so you cannot make a low approach and pull the nose up at a shallow angle. He can shoot back. You have to fly along underneath by quite a bit of distance and with a lot of speed, pull up hard, and fire at the wings/fuselage before breaking off. Because you need distance to pull up, aim, adjust, and fire, you must fly well below the bombers to pull this off. When you come out of your attack run you are vulnerable to return fire (you are now above the bombers), so break away sideways very quickly. Due to the fleeting nature of this kind of attack, heavy cannons are best.