It doesn't take nearly that much ammo, or that many hits to wipe out a flock of bombers, especially B24's. Those are extremely easy to set afire if you put a concentration of fire in the right place.
The first step is to set your convergence effectively, the second is to hit a vital area on your target, from as close to that convergence distance as possible.
Resist the urge to set your convergence way out to a long distance. Like Wolfala says, some people like to stagger their convergence settings, others like to set them all to the same point.
Personally, I like to set them out to around 300yds (I use 275, for a "special" reason), because following a lot of testing in the DA, i think that's about the "optimal" range to set them. I won't say Wolfala's settings aren't good, but I sure wouldn't set wing-mounted guns out any further than that, as it really seems to neuter them for the ranges where they're the most effective. With a setting of 300yds, as the icon counter shifts from D400 to D200, you're right at 300 yards from your target.
When it comes to killing bombers, you need to get a concentrated enough stream of fire, for long enough, in a vulnerable area, without getting yourself shot. This film shows one of the easiest, most effective attacks. I'm higher above the buffs than necessary, but you get the idea. Now, these attacks are from above, at a steep angle, but not steep enough in the "firing portion" of the attack to survive some of the best bomber gunners. You can see the idea though.
Vulnerable areas are wing roots, cockpits, and wingtips (if a bomber loses a wingtip, it's doomed). Once set afire, the bombers will always die, so time your passes so that one set afire "pops" before you attack again, so you have fewer guns pointed at you...
If you're going to look at killing the whole flock, it's also important to attack in proper "order". It's best to kill the right-hand drone first, followed by the left drone, saving the leader for last. This eliminates "warpy" bombers that result from killing the leader first, and also eliminates the slight challenge of shooting the right-hand drone as it shifts positions, which is what happens if you shoot the left-hand drone first.
This film is a bit old, so the flames from the bombers aren't showing. Each bomber is set afire in a single pass, with relatively few rounds. The F4U carries enough .50's that I've gotten 10+ bomber kills in a single hop. The P47 uses the same guns, but has two extras...
http://www.4shared.com/file/wKWoxP0r/B24s_0001.html
