.38 spl. and .357 mag (not the same caliber, but the .357 was spawned off the .38)
Ummmm ... you're wrong. Both the .38 Special and the .357 Magnum are the exact same caliber -- .357. The reason one is called a .38 is because it dates to the black powder era. The first S&W metallic cartridges were heel-based bullets, meaning the bullet was the same diameter as the outside of the case, but was stepped to a smaller diameter that fit inside the case mouth. The .22LR is the last cartridge in common use with this design.
Guns designed to fire heel based bullets were cheaper and easier to make, because the chamber was a simple cylinder. With a bullet that fits entirely inside the case, as all moden centerfire rounds so, the chamber has to be large enough to fit the case, and then reduce in diameter to the throat and bore. Despite the easier manufacture of firearms using heel-base bullets, heel-based bullets faded away because the bullet lube had to be on the part of the bullet that was outside the case. They picked up dirt, which led to malfunctions and jams.
When Smith & Wesson realized that the market demanded inside-lubed bullets, it could have gone either of two routes. It could have made the case large enough that the .375 caliber bullet would fit inside it, or it could shrink the bullet to fit inside the existing case. It chose the latter. Hence, we have a cartridge commonly called a ".38" but which actually fires a bullet that is only 36 caliber, or more precisely, .357.
When S&W introduced the new lengthened and more powerful version of the cartridge, it named it using the actual bullet diameter.
The same basic history explains in large part why so-called ".44" caliber revolvers actually fire .43 caliber bullets. The original Smith & Wesson .44 caliber cartridge used a heel-based bullet (the .44 S&W American). At the request of the Russian government, S&W changed it to an inside-lubed, straight-sided bullet, which became known as the .44 Russian, despite the fact that it was now only .43 caliber. The .44 Russian later was stretched to become the .44 Special, and later stretched again to become the .44 Magnum. All, however, fire .43 caliber bullets.