The drawing shown is of an M8 API, in which the tip in front of the armour-piercing core is filled with a small quantity of incendiary material.
There was also a .50 M1 incendiary, which was a scaled-up and simplified version of the British Dixon ("De Wilde") .303. This consisted of four elements: the bullet jacket which enclosed the whole bullet except for the base; a hollow steel sleeve fitting inside the jacket for the central helf of its length; the incendiary mixture which filled all of the nose, plus the inside of the steel sleeve; and a base plug, usually of lead. To get at the incendiary material while preserving the appearance of the bullet, it would be necessary to drill through the base plug. This would be risky as you might ignite the material. I also don't know how you'd get it out.
Incidentally, they didn't use WP but a mix of 50% barium nitrate and 50% powdered aluminium/magnesium alloy (known as IM#11).