While the .357 Sig isn't a bad cartridge and it does indeed do what it was intended to do, one has to understand that it's a bit of a one-trick-pony. Far and away the most commonly available bullet weight in this caliber is 125gr or thereabouts. While there are lighter 115gr and heavier 147gr offerings, they are not generally popular because their performance is not better, and usually inferior to, the 125gr loadings (115gr loadings overexpand and underpenetrate in soft targets and 147gr loadings don't have enough velocity to offer better penetration against hard targets than the better 9mm and .40 S&W loadings).
The .40 S&W, however, is a more versatile cartridge as JHP loadings are commonly available in 155gr, 165gr, and 180gr weights with 135gr and 200gr loadings not too difficult to find if you know where to look. Likewise, .40 S&W typically performs quite well with bullets in the more common 155-180gr range and allows one to taylor the ammunition a bit more to the task at hand.
To fully understand this, one must understand the history of the .357 Sig and what it was designed to be. The original thought behind the .357 Sig was to duplicate the 125gr .357 Magnum loadings' performance in a semi-automatic handgun. In it's original loadings (and some current boutique loadings) it was able to match the Magnum's ballistics with a 125gr bullet at 1400+fps. Unfortunately, the designers overlooked the differences in the JHP bullets that were useable in a semi-automatics and those used in revolvers. The result was that the original .357 Sig loadings, despite their impressive ballistics, frequently suffered from overexpansion, excessive fragmentation, and underpenetration.
The solution to this was two-fold. First, most manufacturers dropped their velocities by 50-100fps so that most current .357 Sig loadings run at approximately 1350fps from a 4" barrel. Secondly, better bullets (particularly bonded bullets) were designed to better hold together at .357 Sig velocities. While these changes made the .357 Sig perform better than it had in its original loadings, they also made it perform much more like the other popular semi-auto cartridges (9mm, .40, and .45) and less like the .357 Magnum loadings it was designed to imitate (violent expansion with moderate fragementation while retaining adequate penetration).
To my mind, the only real advantage that the .357 Sig offers over the other popular semi-auto cartridges is penetration of certain barriers such as automobile bodies. That increased penetration, however, comes at the cost of lower capacity than 9mm, greater recoil than either 9mm or .45 ACP and a louder, sharper report than any of its contemporaries. Also, it shares the 9mm's weakness against sloped glass due to it's relatively light bullet weight.
While velocity and diameter are the primary factors that play into a bullet's penetration against sheet steel, momentum is more important with auto glass. If we look as Speer's Gold Dot line for example, we see that the .357 Sig loading has lower momentum (24) than any of the three .40 S&W loadings (155gr-26, 165gr-27, 180gr-26). Also, while the .357 Sig does have more energy than the .40 S&W loadings (505fpe), it is only significantly higher than the 180gr loading (419fpe) as the 155gr (495fpe) and 165gr (484fpe) are still quite close to it.
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