A basic understanding of this subject is essential as it is an area where the inexperienced can really show lack of knowledge. Knowing the difference between a 9 x 18 and 9 x 19 mm cartridge, for example, may seem a little insignificant, but it is an area where the unwary can easily be tripped up and made to look very foolish. Having said that, it is a vastly complicated subject, and there are very few set rules.
The first identifier is whether the cartridge in question is referred to in metric or imperial measurements. This generally indicates whether it is of European or British/American origin.
Where British/American cartridges are concerned, the designation is always in inches, and the nought in front of the decimal point is always omitted: for example, a cartridge with a bullet measuring 0.32 in. in diameter would be referred to as a .32". Where European cartridges are concerned, the measurement is always quoted in millimetres, for example, 9 mm. Even this apparently simple identifier is confusing as a number of cartridges are identified by both systems, that is, 9 mm. Short is also 0.380" Auto and 7.65 mm is also 0.32" Automatic Colt Pistol (ACP).
Probably the most confusing part of cartridge nomenclature is the calibre. The calibre (United States - caliber) is basically a numerical approximation of the diameter of the bullet.
This very frequently bears little relationship to the actual bullet measurement, that is, the 0.455" Webley revolver cartridge has a bullet measuring 0.450" and a 0.38" Special bullet measures 0.357". This discrepancy is, however, much more of a problem with older English cartridge nomenclature than it is with modern metric designations: a 9 mm Parabellum bullet is 9 mm in diameter, and a 5.56 mm does have a bullet measuring 5.56 mm.
The nominal calibre is often further identified by a name which can identify it among groups of the same calibre, that is, 9 mm Parabellum, 9 mm Bayard, 9 mm Short, 9 mm Makarov, 9 mm Steyr, and so on. The addition of a name often identifies the weapon for which the cartridge was originally designed; thus, the 9 mm Mauser was designed for the 9 mm 'broom-handled' Mauser, 5.75 mm Velo-Dog for the 5.75 mm VeloDog revolver (designed, as its name implies, for early cyclists and motorcyclists to protect themselves against attack by dogs!) and a 0.32" ACP for the Colt self-loading pistol.
In the European system, it is usual, especially in rifle calibres, to add the cartridge case length to further identify the cartridge, that is, 5.56 x 54 mm, 6.5 x 57 mm. Pistol ammunition can also take this form, although it is not always referred to, that is, 9 x 19 mm is 9 mm Parabellum; 9 x 18 mm is the 9 mm Makarov, and 7.62 x 25 mm is the Russian Tokarev pistol round.
As explained earlier, the case length can also be given a letter to indicate the type of case, that is, 6.5 x 57 R is a cartridge with a rimmed case; 7.92 x 61 RB is a cartridge with a rebated head. This can be even more confusing when a bullet type is appended to the suffix, that is, the 6.5 x 57 RS is a rimmed cartridge with a 'spitzer', or pointed, bullet.
Another suffix appended to the designation of self-loading pistol ammunition is 'ACP'. This merely indicates that it was originally designed for use in Colt self-loading pistols, that is, 0.32" ACP and 0.380" ACP. The letters ACP stand for Automatic Colt Pistol, which is somewhat confusing in itself. An automatic weapon is one in which the weapon will continue to fire 'automatically' until the finger is released from the trigger or the magazine is empty. The correct designation for the pistols for which the 0.380" and 0.32" ammunition were designed is a self-loading or semi-automatic pistol.
In American ammunition, there is often a set of figures which can indicate the year of introduction for that particular calibre of ammunition, that is, 0.3006 is a 0.30" calibre rifle round introduced in 1906, and a 0.30-03 is the same calibre but introduced in 1903. Where it really gets confusing is when the weight of ' black powder' for which the cartridge was originally designed is included, that is, a 0.30-30 is a 0.30" calibre rifle bullet originally designed to be driven by 30 gr (grains) of black powder propellant. Even more confusing, if that is possible, is the system of including the bullet weight into the title, that is, 0.45-70-500, which is a 0.45" calibre rifle bullet propelled by 70 gr of black powder with a 500 gr bullet. What makes this system particularly difficult to deal with is that the majority of these cartridges no longer use black powder, but instead use a much smaller charge of modern smokeless propellant.
In old British sporting and military cartridges, the term 'express cartridge' is often used. This originated with the introduction of a high-velocity rifle and cartridge by the gunmaker Purdy, who designated it the - express train - model. The ' train' part was eventually dropped, and ' express' was reserved for any large-capacity cartridge with a high velocity. For cartridges with even higher velocities, the term 'super express' was also pressed into service. These cartridges were, however, all loaded with ' black powder' , and when ' smokeless' propel-lants came into being, these 'express' cartridges were re-designated 'nitro express'. Realizing that even more power could be extracted from smokeless cartridges, the gunmakers increased the case length and called these new super rounds -Magnum Nitro Express'.
In recent years, the term 'Magnum' has crept into the terminology for pistol ammunition and a 'Magnum' suffix, for example, 0.22" Magnum, 0.32" Magnum, 0.357" Magnum, and so on, is now used to designate a round of much higher than standard velocity.
One other piece of information which can be included in the designation is the nominal velocity, that is, 0.25-3000, being a 0.25" bullet at a velocity of 3000 ft/s. This is, however, unusual, and in the stated case was only used as an advertising gimmick.
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