I recently read about varieties of ammunition that are “necked down”- a larger case and charge of propellent being used with a smaller caliber bullet. For example, .357 SIG is described as a 40 caliber case with a .355 diameter bullet. Now obviously the barrel has to fit the bullet, and guns are traditionally described by the bullet/barrel caliber. So this would be considered a .357 round. But doesn’t the action/ chamber have to be sized to fit the cartridge? Would a gun designed to use this cartridge be unable to use a standard .355 cartridge? Wikipedia’s article suggests that you would need something like a standard 40-caliber handgun and replace the original barrel with the smaller size. In other words, this is blurring the distinction I’d imagined between gun calibers.
It couldn’t use the standard .355 cartridge. The barrel and chamber are two different things. The case needs to fit snugly in the chamber and the bullet has to be the same size as the bore. There are an almost infinite variety of rifle cartridges where someone started out with a popular commercial cartridge and modified it for a different sized bullet, either larger or smaller in diameter. It’s simpler and cheaper than starting out from scratch with a completely new design.
Well, there are lots of strange things in the firearms world, but AFAIK there is no .355 cartridge per se. There are however many that have been created by necking down cartridges that had necks that were of larger diameter, or caliber. For example there’s a 25-06 made from a 30-06 case. 25 caliber bullet instead of 30 caliber.
The 17HMR, a rim fire round, is a .22 magnum round necked down for a 17 caliber bullet. Smaller bullet, but much higher velocity.
Someone has an idea and manufactures a barrel and chamber to match their idea. Then they order, or manufacture the dies required to resize the cases that the new round will be made from.
If a lot of folks like the new round a manufacturer will tool up for commercial production.
Somewhat simplified, but that’s the basics.
As noted, people invent new cartridges all the time for various reasons. The vast majority of them aren’t commercially successful even if that is a goal but some are. Roughly speaking, the person that invented the zippy 25.06 took a 30.06 cartridge and figured out a way to shrink the end of the brass shell down at the end so that it holds a .25 caliber bullet instead of a .30 caliber bullet. A 30.06 chamber can still hold the brass shell just fine roughly speaking because most of it, except for the neck is the same shape as it was before. However, a 30.06 barrel is too big for a .25 caliber bullet so the inventor has to find a way to mate the existing chamber to a new type of barrel. There are lots of ways that may be possible including inventing and manufacturing a custom barrel for your new gun for it to accommodate your new combination of chamber/barrel/cartridge.