This is from WW1 Aircraft Performance by Anders F. Jonsson, pp.113-115:
The British were early adopters of these new alternative ammunition types, with the earliest round being developed already in 1902. This was an explosive nitro-glycerine filled bullet known as the Pomeroy, aptly named after the New Zealand born engineer James Pomeroy who invented it. Another British incendiary round (filled with phosphorus) that made an early appearance in the war was the Buckingham, developed by James Buckingham in 1914. Yet another British invention was an explosive bullet that was introduced in 1917, and which was designed to burst between the outer skin and the gas cells in airships, the design of which was attributed to Commander Frederick Brock...
The Germans were not as quick as the British to adopt alternative ammunition types, but in 1917 there were two phosphorus rounds used: The first was the s.Pr.(H) which was a steel-jacketed round containing phosphorus with only minimal lead content. This bullet had small holes on the sides, sealed with low-temperature solder that opened on firing, meaning that the round was already on fire when it hit the target. This combination of higher incendiary content and early activation mean that it was ideal for attacking balloons. There was also a second, more all-round German incendiary round, which was also suitable for combating aircraft. This bullet contained a steel core for penetration and a phosphorus filling for fire effect, and it was simply designated the "F" as an abbreviation for Flugzeugbrand mit Stahlkern (literally translated to "aircraft fire with steel core")...
The above information on the German efforts during WW1 to produce better bullets for aerial combat comes from a treatise by Achem Engels, which at the time of writing can be downloaded from the Museum für Flugzeugbau und technische Geschichte. "Die Standard-Bewaffnung der Kampfflugzeuge im Ersten Weltkrieg, Das gesteuerte L.M.G. 08/15". [It would be interesting if any of our German-speaking members could try to find this.]
Of note is that the incendiary bullets used on both sides were fickle beasts and were known to spontaneously ignite with little to no provocation, especially after longer periods of storage and if subjected to elevated temperatures. This resulted in the introduction of special cooling arrangements like the removal of engine cowlings, which can sometimes be seen on late war German scouts like the Fokker D.VII, where the side cowlings have been removed to remedy the insufficient cooling provided by the standard louvers. [Of course, this was also sometimes done to improve engine cooling.] On the British side, there was a preference to use the incendiaries only in the Lewis MG, since the open bolt firing system ensured a cool environment for the fickle incendiaries even after prolonged firing. This ensured that the over-wing Lewis remained popular with some pilots throughout the war, even after the introduction of reliable synchronized MGs fring through the propeller arc.
Also, I seem to remember there was some prohibition about firing incendiary ammo at enemy personnel, though I'm not sure that ever stopped anyone.
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