Metric Designations

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Metric and American
Metric and American


Q How do the Europeans measure a cartridge? When I was in the Australian Army, we shot the 5.56x45mm and 7.62x51mm NATO round. I know the first figure is the calibre which is .224 and .308, but is the last figure the overall length of the cartridge or the length of the case?
 Albert Barnes

A You could have got the answer by measuring the length of the case and the loaded cartridge, however, the second number in a metric designation is the length of the case in millimetres. Regarding the 7.62x51mm, "7.62" is the metric diameter of the bore of the barrel and "51" is the length of the brass case without the bullet. Translated into inches 7.62mm = .300,which is the barrels minor diameter, while the groove diameter is .308" the same as the actual bullet diameter. Ditto for the 5.56x45mm. European makers generally use the barrel's land diameter to identify the cartridge's calibre while most American makers name cartridges after the barrel's groove diameter, but not always, as witness the .300 WSM. About the only thing you can be sure of about cartridge designation is that it is real mess.

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