The .357 Smith and Wesson
is a rimmed revolver round that was introduced in 1935
as a more powerful version of the .38 Special
Smith and Wesson cartridge, precisely because men like large amounts of kinetic energy
. The larger the amount of kinetic energy produced, the better. Note that the .357 mag and the .38 special both use a .357" caliber bullet and .38 special can be safely fired in .357 mag revolvers, try to do it the other way around and I will personally deliver your darwin award
to your next of kin
Touted as a super cartridge, and indeed it was in its day, there were tall tales of a .357 magnum bullet splitting a car's engine block in half, which is not true of course.
Typical safe maximum loads for this cartridge will generate velocities around 1,900 feet per second when using a 110 gr (the lighter extreme) bullet producing 209,000 power factor. A heavier 180 gr bullet can safely be pushed to about 1,308 feet per second which produces 235,440 power factor.
The .357 Sig is an entirely different animal in that it is a bottle necked rimless round, typically used in semiautomatic pistols. It is not a true .357 as it actually uses a .355 caliber bullet, the same as the 9mmx19 nato or parabellum cartridge. While it is indeed a "magnum" compared to the anemic 9mm the designation "357 Sig" was most probably the handiwork of a suave marketroid working for Sig Sauer. In NATO measurements it would be a 9mmx22 or thereabouts. Coonan arms makes a 1911 pistol chambered for the .357 mag and IMI makes the famous Desert Eagle which was actually chambered for .357 mag in its original design.