For example, why 5.56mm and not just 6mm? Why 7.62mm instead of just 8?
In: Technology
There are tons of calibers and they vary greatly. But sometimes it’s that stuff is designed with the inch measurement in mind and sometimes it’s not. There are some that are whole numbers in the metric system, for instance the 10mm pistol round. Which BTW a shortened version of the 10mm is the 40S&W.
Another thing is that sometimes a particular size is just better suited for a weight and ballistic coefficient. When you start looking at rounds that will be used at a longer range that makes a huge difference.
[https://en.wikipedia.org/wiki/Table_of_handgun_and_rifle_cartridges](https://en.wikipedia.org/wiki/Table_of_handgun_and_rifle_cartridges) gives a listing of a lot of the major rifle and pistol rounds. The older it is the more likely it is to be of an inch variant and newer stuff that is designed to use an existing bullet will be as well.
Also a lot of times it’s just in a name. Can’t have 10 different callibers that all are 10mm now can we. So we can call one a 10mm, one a .40 and another a 10.1mm and they can all use the same bullet but technically be a different round.
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