Usually, a belt was loaded with a repeating mix of ammo, such as API, to punch holes and start fires; then incendiary was used to set fire to ground structures, etc. The exact mix of the cartridges in the belts was sometimes dictated by unit or squadron policy, though some pilots chose the mix usually used in their aircraft.  Information about this has been lost and the author would appriciate hearing from any ordancve types out there who have something to say about what was actually being done.
America's .50 Caliber
Do-All, Kill All Ammo
The half inch diameter .50 caliber (12.7mm) slug we used to win the War was just big enough and heavy enough that it could be readily adapted to many purposes without compromising either  velocity or accuracy.
Essentiially, there were five basic rounds that differed only in what they carried inside their pointy noses, and they were identified by color codes.
 * BLACK TIP: M2 armor piercing, would penetrate all, or most, airborne armor plate.
 * SILVER TIP: M8 armor piercing incendiary (API), would penetrate and produced white-hot flame on impact.
 * BLUE TIP M1 incendiary, primarily designed to set fires.
 * ORANGE TIP: M1 tracer, usually mixed at least one in every four rounds for sighting purposes, but some aces preferred to have no tracer so the enemy wouldn't know they were being fired on until they had been hit.
 * RED TIP: M1 or M21 tracer, a later version of the orange.

Pages 62 - 68 of October 1998
F L I G H T   J O U R N A L

A photo of Browning Machine guns as used in the P-47D
Photos by Jeff Ethell via Bert Kinzey
Below are approximately 450 rounds of .50 caliber ammo in the left wing of a P-47D
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