Sorry for the delay, just seen this one....
When incendiaries were first used in WW1, they were generally of the type which ignited on firing and burned all the way to the target. This had the advantage of leaving a smoke trail so they were effectively 'incendiary/tracers' and sometimes described as such. The British had these in use in the BoB (as the B.IV) alongside the new 'De Wilde' (B.VI) which was different in that it burst into flame on impact with the target. This was obviously more efficient as it didn't waste the incendiary compound. It also helped that the ignition caused a bright flash on impact, so you knew when you were scoring hits.
The US .30 and .50 incendiary ammo was based on the British 'De Wilde'. They were purely filled with incendiary compound - no HE - but the fact that they burst violently into flame on impact gave them the appearance of explosive rounds.
There were some US explosive .50 rounds developed in WW2 but they remained experimental and were never used. The Soviets, Italians, Germans and Japanese all used HE shells in 12.7-13 mm calibre weapons (the Japanese used them in rifle-calibre MGs as well).
Tony Williams: Military gun and ammunition http://www.quarry.nildram.co.uk][/color]website and discussion
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