The vast majority of the time, 38 is a reference to the 38 special. It's actual caliber is .357 just like the 357 magnum. In fact, the 357 magnum is based on the 38 special but operates at a much higher pressure and it is a bit longer, primarily so that it won't fit in a 38 special revolver that wasn't designed for the higher pressure.
357 Sig is a very different cartridge than the 357 magnum. The 357 magnum is a much longer straight walled and rimmed cartridge designed to work in a revolver. The 357 Sig is a rimless bottle necked cartridge based on a necked down 40 Smith & Wesson cartridge and designed to work in a semi-auto pistol. It's caliber is actually .355, the same as a 9mm.
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