Riﬂe cartridges don’t just coalesce out of thin air. Most of them “evolve” from existing cartridges. Of course, it takes a creative human to force that evolution by changing the shape of the cartridge.
The .243 Winchester, for instance, was invented when someone reduced the neck diameter of the .308 Winchester to hold a .243-inch diameter bullet. The rest of the brass case was basically unchanged. This meant the powder reservoir (volume) remained while bullet diameter and weight shrank. Instead of pushing a 150-grain bullet 2,800 fps, the powder accelerated a 100-grain bullet to nearly 3,000 fps.
But why not just reduced the weight of the .308 bullet from 150 grains to 100 grains and throw that 3,000 fps? You can, but such a bullet is short and fat, meaning it’s ballistically inefﬁcient, wasting most of its energy pushing air. The longer, narrower 100-grain .243 bullet resists air drag, shoots ﬂatter and retains more downrange energy.
Because of such ballistic performance differences in bullet weight and shape, shooters manipulate cases to create all sorts of new rounds. The .308 Win., released commercially by Winchester in 1952, is basically a shortened .30-06 case. The .30-06 Springﬁeld case sprang from the 7x57mm Mauser case.
As a parent case, the .308 Win. has been re-shaped to create not only the .243 Win., but also the the .260 Rem., 7mm-08 Rem., .338 Federal and .358 Win. Wildcatters have also made it into a .224, .257 and .277, both those never caught on to become commercial rounds.