This is from 'Rapid Fire':
"The earliest machine gun cartridges were designed to use black powder (gunpowder) as a propellant. This is a simple mixture of saltpetre, sulphur and charcoal, and has been used for centuries. It is not ideal as a propellant, however, because of its inefficient burning characteristics and the large quantities of smoke and other residues generated.
Improvements in chemical science in the late nineteenth century led to the development of smokeless powders, the first of these entering service in France in the late 1880s. Other nations soon followed suit, with the British version being known as "cordite" because it was extruded in fine strings resembling cords, which were bundled together for insertion into the cartridge case.
There are three different types of smokeless propellant, known as single-base, double-base and triple-base. All are based on nitro-cellulose with nitro-glycerine (or equivalent) and nitro-guanidine as other major components in the more complex versions, triple-base including all three. Cordite is a double-base type.
The increasing complexity of the propellants is due to the constant search for the ideal combination of characteristics. These include maximum power for a high muzzle velocity, moderate pressure and temperature to minimise the stresses and erosive effects on the gun, as little fouling and corrosion as possible, and a minimum of smoke and flash at the muzzle. Many of these desiderata are mutually exclusive so each propellant is a compromise. As with other chemical ammunition components propellants must be insensitive to rough treatment, provide consistent performance over a wide range of climatic environments and be tolerant of storage for long periods in poor conditions.
The performance of a given cartridge depends upon the maximum pressure which the gun is designed to accept. Pistols and shotguns, for example, are usually only intended to work at relatively low pressures while rifles, machine guns and cannon are generally much stronger, so cartridges for them can be loaded to higher pressures and therefore velocities. Not all guns are the same, however, and some can take much higher pressures than others. This is usually determined by the strength of the mechanism which locks the bolt or breechblock to the breech at the instant of firing, as will be discussed in the next chapter.
Propellants today are generally prepared in the form of grains or small pellets. The precise chemical composition, and the size and shape of the grains, will vary from one cartridge to another in order to provide the power and pressure characteristics suited to the gun."
British aircraft ammunition in WW2 was available with both cordite and 'nitro' loadings (the latter being recognisable by the letter 'z' added to the ammo code on the headstamp).
Tony Williams: Military gun and ammunition
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