Extracts from 'Flying Guns - World War 1: Development of Aircraft Guns, Ammunition and Installations 1914-33':
"Reliability was a significant problem in First World War aircraft guns, and the main cause of failure was poor quality ammunition. In the British 'Handbook of Aircraft Armament', the notes on the Lewis gun contain the following:
Examination of Ammunition
Every round should be carefully looked over for dents, deepset caps, defective bullets, split cases &c., before it is placed in the magazine.
The best service test applicable to discover defects in shape is to use a spare Lewis gun barrel and drop each round into it in order to see that the cartridge enters freely. The rims should be examined to see that, as far as possibly by eye, they are not too thick.
Several cases have been discovered in England of the cartridge containing insufficient or no charge, so that it would be convenient if the N.C.O. or man in charge of guns were able to test the comparative weight of each cartridge with a good one.
If possible U.S. ammunition should not be used as it has been found defective in various respects at this school."
"A major contributor to synchronisation problems (and gun reliability generally) was ammunition quality, which tended to be variable during the War. Pilots frequently carried a mallet with which to hammer the loading lever in order to chamber a recalcitrant cartridge. In an attempt to resolve this, the British introduced in 1917 "Green Label" (or "Green Cross") .303 in ball ammunition specifically for synchronised guns. This was taken from standard production lines, but carefully selected from batches which complied with tighter manufacturing tolerances and gave reliable ignition. This proved successful and was followed up in 1918 by establishing special production lines to make high quality ammunition for this purpose. This was known as "Red Label" (also as "Special for RAF, Red Label", "Special for RAF" and finally "Special") and ball, AP and SPG tracer ammunition were produced."
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