A WW2 vintage round or a modern round? To the best of my knowledge, the US didn't adopt a 'explosive-tipped' .50 round until the mid 1980s (and even then, by acquiring a foreign developed round).
Standard .50 cal rounds at the beginning of the war were the M2 ball, M2 AP and M1 tracer. Typical belting was 2 ball - 2 AP - 1 tracer
A new M1 incendiary round was introduced during 1942, as was the M2 tracer. These largely replaced the M2 ball and the M1 tracer in belt configurations. The M10 tracer was introduced a little later than these, although I can't find an exact date of service entry at the moment, and rapidly replaced the M1 tracer.
In 1943 the M8 AP-Incendiary and M20 AP-Incendiary-Tracer were developed, with both introduced sometime around very late 1943 or early 1944 (EDIT: Tony Williams gives "Spring 1944" for M8 service entry). These basically replaced everything else except for M2 AP. Standard belt configurations were typically comprised of AP, API and API-T, although it seems that incendiary rounds were more favoured in the PTO (possibly because of a lack of armour on Japanese aircraft).
There was also the M23 Incendiary round, which didn't enter service until just before VE Day.
It's not me that was making the argument, it was the RAF.