Best to shoot inside 300 yards or closer.
Use the .50 cal's to "range" a target, use page up to get more view for pulling lead shots.
Keep all guns converged at around 300, try setting the cannon at longer ranges.
The Thing i have noticed is no matter what the guns are set to, if you keep a bead on your target for more than 1.5-2 seconds while fireing all guns i would say you have a 60-80% chance of getting 37mm cannon hits on your target.
Most times i get multiple cannon hits on someones ac before they vanish.
Like most things practice makes perfect, and some more food for thought is the fact the germans 30mm ballistic is even worse than the american m4 37mm.
I think the best comparison for a more rapid fireing 610m/s weapon "cannon wise" would be the first zeke.
its guns match closely, so try that plane out for a more "rapid fire" aspect.
When you get good hitting with the relativly yet high rate of fire for the zeke, you may find the 39's 37mm more adaptable.