French Incendiary Ammunition during WW1

Observation balloons were very important for directing fire during of the First World War, and this made them prime targets for aviators of all nations. Many methods were used to destroy the balloons, including hooks, bombs, explosive arrows and rockets. However, by mid-1917, most nations were using various forms of incendiary ammunition. The British had Buckingham incendiary rounds as well as the explosive Brock and Pomeroy rounds. Then there was the British RTS round, which was both incendiary and explosive, and fielded in June of 1918. The Germans used incendiary Ph- (as in phosphorus) ammunition.

The French Aéronautique Militaire also required incendiary ammunition to engage observation balloons, and they found that the best way of providing enough fiery potential in a round was to increase the caliber. .303 was seen as not being a big enough caliber, so in 1917 the French introduced an 11mm Vickers machine gun, the so-called Gras Vickers, that could fire a modified version of the old 11x59R Gras rifle cartridge which contained a Desvignes Mark XI incendiary bullet. The Desvignes was actually a tracer bullet, but the size of the round enabled a long and powerful burn. Despite the caliber, the bullet was lighter, and it thus had greater muzzle velocity, in this case some 600 m/s. The Gras Vickers was mainly fitted to SPAD XIII scouts, often with one 11mm and one .303 Vickers in the nose.

The Gras Vickers was also used by the United States Army Air Service (USAAS), mainly on Nieuport 28 aircraft, and it was introduced by the Americans in late 1917. The Americans converted Vickers machine guns that were chambered for a Russian 7.62x54R round as part of an order that had been cancelled following the Russian Revolution, and they also introduced a heavier bullet with a weight of 17.5 g. However, this was not popular due to the heavier recoil, which caused excessive vibration while firing the gun, but the effect of the High Explosive 11mm round was still in demand, especially against ground targets, so the round remained in production even as rifle-caliber incendiary rounds became available to the USAAS. Some 500,000 11mm rounds were manufactured by the Americans up to the early 1920s, when the round was discontinued. The Gras Vickers is claimed to have been influential in the design of the .50 Browning M2 machine gun.

Belgian ace and balloon-buster Willy Coppens had the Gras Vickers fitted to his Hanriot HD.1 scout No.17, one of several Hanriots that Coppens used. He held this conversion in high regard, and at least one other Hanriot may have been armed with a Gras Vickers, although the Belgians did not manufacture the gun. A few Gras Vickers were also used by the RFC, RNAS and eventually the RAF. The last official use was in Yemen in the 1950s.


Sources:

www.theaerodrome.com/forum/showthread.php?t=24505 
worldatwar.net/chandelle/v2/v2n1/hd1for.html 
www.histavia21.net/amaviapag/11_gras_avia.htm 
riseofflight.com/forum/topic/29740-11-mm-balloon-gun/ 
http://www.warrelics.eu/forum/world-firearms/ww1-colt-vickers-11mm-gras-ammo-4103/ 
http://www.gunauction.com/buy/5808806