Two Naming Traditions
Cartridge names descend from two broad measurement cultures: the metric system used across much of Europe and the imperial system rooted in British and American practice. A name is a label, not a specification, so learning how each tradition builds its labels prevents costly confusion when you compare entries in a reference table.
Metric Naming Convention
Metric names most often pair the bullet diameter with the case length, both expressed in millimeters and joined by an "x". In 9x19, the 9 is the nominal bullet diameter and the 19 is the case length. This format packs two genuinely useful dimensions into a compact name, which is why metric designations tend to feel systematic and predictable.
Imperial Naming Convention
Imperial names use fractions of an inch, typically hundredths or thousandths, and frequently append a word or number tied to history rather than geometry. A ".308" refers to roughly three hundred eight thousandths of an inch of bullet diameter. Because a suffix may honor a designer, a year, or a marketing choice, imperial names carry information that is not strictly dimensional.
Nominal Versus Actual Diameter
A crucial subtlety in both systems is that the number in a name is nominal, meaning it is an approximate label rather than the measured bore or bullet size. The ".38" and the ".357" families, for instance, share a very similar actual bullet diameter despite different names. The authoritative figures always come from standardized dimensional drawings, not from the digits in the popular name.
Same Round, Two Names
Many cartridges carry both a metric and an imperial name because they were adopted on both sides of the Atlantic. This dual identity is common and expected, so a reference library will often cross-list the two labels to show they describe one specification.
Worked Example
Consider 9x19, also widely written as 9mm Luger. The metric name tells you directly that the bullet is about nine millimeters across and the case is nineteen millimeters long. Its imperial-influenced alternate name, 9mm Luger, honors designer Georg Luger and encodes no dimensions at all. Both names point to the identical standardized cartridge, and a good table lists them together so a reader searching either term lands in the same place.
A Common Misconception
People often assume the number in an imperial name is an exact bullet measurement. It is not. Names like ".38 Special" are historical and nominal, and the true bullet may measure closer to .357 inches. Always treat the name as an identifier and consult the dimensional standard for the precise figure.