American fashionista.

When people say .380 -- it's a common abbreviation for ".380 ACP", which has these dimensions: http://www.handloads.com/images/cartridge/380acp.jpg

And as you can see... none of those dimensions are actually .380". .380 is just a descriptive name it was given to distinguish it from other similar sized cartridges. There are tons of different cartridges that share the same bullet diameter, so just naming the diameter is not enough to actually distinguish a cartridge. For that reason, cartridges are given names that describe the entire design. Actually this same cartridge has lots of other historical names as well, that were used in other places around the world, such as:

.380 Auto, 9mm Browning, 9mm Corto, 9mm Kurz, 9mm Short, 9×17mm and 9 mm Browning Court

https://en.wikipedia.org/wiki/.380_ACP

The same sorts of naming quirks apply to other cartridges as well.

Tl;Dr -- they're named out of convenience, it's not a full technical specification, nor are they "accurate" measurements.

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