[stephen f. polyak]

The US used at least five type-classified caliber .50 tracer cartridges in WW2. One was an armor-piercing type (M20). That bullet is 2.31 inches in length. Its weight without tracer filler is 610 grains. The rear end of this bullet tapers (like yours). The nose marking was a red tip followed by a silver band.

The other four bullets had a lead slug in the nose and carried more tracer filler in back. Those bullets were 2.4 inches in length. Their weight minus tracer fill was less than 500 grains. There is no taper to the ends of these four. Nose tips are painted red or orange.

From the picture you provide, the overall shape, end taper, and size and location of the cannelure (narrow circumferential grove) would indicate a US cal. .50 round. If the bottom's open and the diameter of the opening is less than 50 percent of the base diameter, an API-T would be a good bet.