For example, why 5.56mm and not just 6mm? Why 7.62mm instead of just 8?
In: Technology
The 5.56mm NATO cartridge was in fact designed using the metric system. The bullet ended up as exactly 5.70mm in diameter, or in imperial it would be .224″. The designation of 5.56mm is from the diameter of the barrel which is smaller then the bullet in order to create an airtight seal around the bullet and allow the rifling to cut into the bullet.
7.62mm is exactly .30″ which were the caliber for the earlier rifles that these cartridges are based on. And even though most .30″ rifles are bigger, some are even over 8mm they were all called 30 caliber which were converted to 7.62mm. It should also be noted that some countries did call these 8mm instead of 30 caliber, for example the 8mm Label and the 8mm Mauser from the French and German manufacturing respectively. However these did not stick around. On the other hand the 9mm Parabellum round have kept its name until today.
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