I've checked the damage values for the various cannons. Whether these values are accurate relative to each other, or not I don't know and it can always be good to discuss. First the blue box shows the damage per bullet caused for the various guns. For some reason the damage value for the 37 mm M4 cannon of the P-39D/Q is only 60% that of other 37 mm cannons, and it's lower than the 30 mm cannon of the Bf 109K-4. Why this is I do not know, and I have not done any research regarding the hitting power of various ww2 cannons. In the red box you can see the actual hitpoint values for the different plane parts of the average fighter in Warbirds (I've summed them all up and calculated the average number). The damage is transferable, so if you destroy the elevator, the remaining damage will go into the vertical stabilizer etc. I've also included the hitpoints for the A6M Zero that you encountered.
As we can see the A6M Zero, which was fairly large but had a light and fragile structure, has generally lower hitpoint values than the average fighter. For an average fighter you would often need two 37 mm cannon hits to the fuselage or wing to destroy the airplane, or one 37 mm hit and some machine gun bullets. The elevator and rudder are gone in a single 37 mm hit, but the vertical stabilizer would be intact unless also hit by a few machine gun bullets. For the A6M Zero both vator/rudder + corresponding stabilizer would be destroyed by a single 37 mm hit. Also the A6M's fuselage would be destroyed by a single 37 mm hit. It's wing can just barely survive a 37 mm hit, and will go poof by a few machinegun bullets extra.
When it comes to the P-39D/Q's 37 mm M4 cannon however you would often need three 37 mm M4 hits in the same part to down any fighter. If your hits are spread out among the plane parts then you could hit it with many M4 shells without causing any fatal damage. Again I don't know why the M4 is set at a lower damage value than the other 37 mm cannons. If you score two M4 cannon bullet hits in the right wing, two in the central fuselage, two in the rear fuselage and two in the right wing, then the enemy fighter will be very fragile and go down in a few more hits from machine guns, but would not have lost any integral part from those eight M4 cannons hits. The Zero would be somewhat easier to down but still follows the same principle.
Especially when spraying an enemy with machine gun hits, it makes a very big difference if you hit all those bullets in the same spot, which will cause the loss of that plane part, or if the hits are spread out over the enemy fighter's whole surface, at which point the various parts will have been weakened, but not yet any damage has been taken. Try to always see where your bullets are hitting home and try to aim your future bursts at the same parts that have been previously hit.
<S>
/Robert
.