Short version of it is most of these names originated a century or longer ago in various parts of the world using different standards, the names have just been kept the same or shortened to the form we have today.
Take 9mm for example. Properly, it should be known as 9x19mm Parabellum, and was designed at the very start of the 1900s. There’s a billion different “9mm” cartridges, some even with 9mm in the name, but 9x19mm Parabellum is so common everyone just calls it 9mm, because any other 9mm cartridge you could get is rare enough you have to special order them (like 9x18mm Makarov).
It was designed in Europe, so it uses mm. It has the same bullet diameter, but because .357 was designed in the US, it uses fraction of an inch in its name. At this point, they’re essentially just used as names and not measurements, so they stay how they are. The distinction is important because they’re completely different rounds and are not interchangeable, here’s a pic of them side by sode:
Short version of it is most of these names originated a century or longer ago in various parts of the world using different standards, the names have just been kept the same or shortened to the form we have today.
Take 9mm for example. Properly, it should be known as 9x19mm Parabellum, and was designed at the very start of the 1900s. There’s a billion different “9mm” cartridges, some even with 9mm in the name, but 9x19mm Parabellum is so common everyone just calls it 9mm, because any other 9mm cartridge you could get is rare enough you have to special order them (like 9x18mm Makarov).
It was designed in Europe, so it uses mm. It has the same bullet diameter, but because .357 was designed in the US, it uses fraction of an inch in its name. At this point, they’re essentially just used as names and not measurements, so they stay how they are. The distinction is important because they’re completely different rounds and are not interchangeable, here’s a pic of them side by sode: