Hi Max,
Here a few things to consider, in no particular order:
- Spreading 50 cal. convergence may get you a few more hits, but will not get you more kills. As Spatula said, it's kinetic energy, so closer and concentrated will do more damage, giving you the kill instead of the assist. We're competing for kills.

- Icon distances are in 200 yard increments, so the instant you see the distance change from 600 to 400, the bogey is 599 yards, not 400.
- You may want to fine tune your aiming point. Fuselage hits don't do much damage, but wing, tail, pilot and engine hits do.
- Try some closer convergences - maybe 300-350 yards. Pull up a target - you can do it in the main arena (".target xxx" where xxx=yards) - and see where you're hitting at different distances, e.g 200, 400, 500, 600, 800, etc.
- You may just be getting 1/2 of your guns (right wing guns only) landing on a plane outside your convergence. Consider that your plane is not always trimmed perfectly and that flying at full power is applying torque that you have to counter with right rudder. Work that rudder.