Doesn't really matter. When you're 400 behind your foe, and you are set (in convergence) at 400 then you will be ready to rock and roll. The bigger question is what do you set your convergence for 50's & 303's, versa Cannon rounds. Be advised, cannon rounds are heavier and drop faster than the lighter rounds. My point being, you cannot set lighter rounds & cannon rounds to the same convergence.
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Rokit
Technically, all rounds drop at the same rate due to gravity. Its the speed of the round that determines how much it drops over a certain distance. Faster rounds will appear to drop less because they cover a greater distance than the slow rounds do in the same time it takes both to hit the ground. Yeah..im nitpicking.
When it comes to setting convergence, If im in a plane that has a mix of cannon and machine gun, I usually base my machine gun convergence on whatever I have my cannon set at. This ensures i have maximum damage at my ideal cannon firing distance. Some people will set their machine gun convergence at a much higher distance so they can use them to get guys to rev when they are running, and then when they rev, they can use their cannon at a closer distance. It really depends on how you use your guns.
If you have nose mounted guns, IMO its best to set them at a very high range. That way you can hit up close and far out.