The .357 Sig
was an attempt by the engineers at SIG arms
to create an autoloading pistol
round that reproduced the ballistic characteristics of the venerable .357 Magnum
. Their success or failure in this regard is still debateable.
The .357 Sig is not a true .357 as it does not use a .357" bullet, rather it uses a .355" (aka 9mm) bullet in a .40 S&W case necked down to accomodate the smaller bullet.
While it does generate higher velocities than an ordinary 9mmx19 it is usually a hundred or so fps below true .357 Magnum velocities. It is quite safe to say, if you want magnum performance then use a magnum revolver and fire magnum rounds. Although to be fair, the improvement of muzzle velocity in the .357 Sig is considerable and if not for the higher price and less availability it could just as easily have been as popular as the 9mm.
D.E. Watters could not have said it better when he said that "one might wonder if the .357 Sig would be as popular as it is if it were named the 9x22 Sig."(9mmx22 would be it's designation in standard NATO measurement nomenclature)
Come to think of it, maybe it wasn't the engineers that made this round, sounds more like the work of the marketroids.