One is thousands of an inch, which is a typical unit of measure for milling operations. As the other person in this chain has said, ballistics is a weird game. End of the day, m(ass)*v(elocity)=f(orce), and the goal is getting f to a useful value. At that level, big and slow and small and fast have the same outcome. But, because ballistics is bastard, other factors start to interfere.

For example, If you want to shoot farther, you can either go faster, or be lighter. Going faster requires more powder in the case, which creates a different profile for the round and changes the acceleration profile during firing (there are several rifle rounds over the years that have approximately the same load and bullet, but are ever so slightly different shaped, to change how the powder burns off and how it accelerates down the barrel)

Being lighter reduces your final impact force, and can impact stability mid flight (leading to alterations to the length and profile of the round itself)

Closer to the first point and tangentially related, the US has been a capitalist hellscape for a while, and has numerous instances of a company making a firearm using a new round because they did the math and said this is what we want and what you should want, followed by everyone else making more or less a clone of it if the gun in question sells well. Due to copyright it can't be identical, but as math goes, there a lots of ways to reach the same outcome.

While the diameter is the same the heights and sometimes shape of the the bullet is a bit different. A .357 is significantly taller than a 9mm. This prevents you from using the bullets interchangeably even though the diameter is the same.

Fortunately, me and the homies went shopping before this went into effect:

7.62x51 sure. 7.62x25 tokarev no big deal

Hey Tokarev was a thing

Let me introduce you to the obrez.

It's not that bad, but I can see why Walmart stopped selling 223mm and 357mm, because the weight of a single one of those will surely break a normal shopping cart when placed inside.

so now where am I supposed to get this?

You can order yours at a naval shipyard of your choice. Unfortunately, due to high demand and supply chain issues, delivery might take a couple years, though.

But highest kill ratio per round!