I'm building a Trumpeter 1/32 F4U-1D (Okinawa circa. 1945) and it can be built with the access panels for the wing guns open or closed. The cartridge belts have tiny little bullets molded at the ends of the cartridges and I'm thinking about (notice I said "Thinking About"; they are pretty tiny!) trying to paint the bullet tips the proper colors for the ammunition type. Not sure of the mix though.
If I remember right, please correct me if I'm wrong, tracers were about 1 round in 5. But what about the other cartridges? Were they AP, incendiary, ball, or what? It may have varied at the whim or the armorer/pilot/CO, I just don't know.
Here is a link to the different types of ammo for the "Ma Deuce":
http://www.globalsecurity.org/military/systems/munitions/50.htm
but they don't say anything about the mix.