>>64385054
because the germans had their machine gun ammo 7,92x57mm, and they just thought "Yeah, it's a good bullet, but the gunpowder amount is too big. So just cut the lenght and gunpowder amount down to the levels where it's manageable, while keeping the rest of the ammo the same.

And that's it. That's how "intermediate" cartridges were born. It wasn't the full sized machinegun/hunting rifle bullet, nor it was the smaller pistol bullet. it was something in between. It was a rifle ammo that was just shortened and reduced in power, so it could be "just rifght for one-man automatic gunner.