Quoted:
Here is something that never made sense to me:
A 9mm and a 380 Automatic are the same diameter. But, 9mm = .354" not .380"
[Edited To Add]
.308" = 7.8232mm
.357" = 9.0678mm
.380" = 9.6520mm
7.62mm = .3000"
9.00mm = .3543"
Again another Caliber that doesn't make any sense. Is there some kinda conversion for Inches to Caliber.
But, 9mm is 93% of 9.65mm, while 7.62mm is 97% of 7.82mm
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I believe there's no need to convert inches to caliber. It's two different ways of saying the same thing.
30 caliber = .300"
I think some of the confusion comes from what a cartridge is eventually called.
The 38 special and .357 magnum is a good example.
IIRC the case diameter on both is .379".
Someone probably decided to round off the number to the nearest 1/1000 of an inch, and bingo 38spl.
When the .357 magnum was developed in 1935 by D.B. Wesson, who also was responsible for the 38spl in 1908, it was decided to refer to the caliber by the diameter of the bullet, .357".
38spl- case diameter = .379", bullet diameter =.357".
357 magnum- case diameter = .379", bullet diameter = .357".