It's good to know the history of firearms, it helps you know why things are, how they are.
The .38 special was a very old cartridge design and was set up to use blackpowder, and had a lot of extra space in the cartridge, when loaded with the new smokeless powders.
By the 1930's A firearms enthusiast and writer named Elmer Keith decided to start experimenting with his guns to get more out of them.
He was using more modern firearms, made of more modern steels that could take much higher pressures than the old guns designed around the black powders, and getting much higher performance out of them. He wrote about what kind of performance he was getting, and people were interested.
He used the old .38 special cartridges with the extra space in them in these guns though, because he wasn't a factory with unlimited resources, just a normal guy who used what he had on hand.
There was so much interest in these new experimental cartridges, that they went mainstream.
Engineers aren't stupid though, and they new that if they just putting a warning on the box not to use these new cartridges loaded with very powerful loads, people were going to start blowing their old weaker guns up, and their hands off. So, they decided to make the new ".357 magnum", only 1/8th of an inch longer than the original .38 special. That way, the new cartridges would not chamber in the old guns, they would stick out just a tiny bit, (1/8th of an inch) and prevent your closing the double action revolver or advancing the cylinder on a single action revolver.
You would still be able to use the old .38's in the new guns, and manufacturers were still able to use almost all the same tooling to make those guns and cases, with just a small modification to make the chambers and cartridges 1/8 inch longer, and no other changes.
Thus, the magnum was born. The word magnum however, is much older, simply referring to a larger than normal bottle of champagne, wine, or spirits.
Make sense now?
