As a cartridge, .357S offers little advantage vs. its drawbacks in a short barreled, lightweight pistol.
My first .357S was a factory slide/barrel conversion for the Keltec P40 around 1996. That was a monster to shoot from an 18 oz. gun, and parts broke quickly. The project was discontinued in less than a year. Granted, it was a Keltec, OK, I get it. But other manufacturers went down the same road with similar conclusions.
Maximum SAAMI pressure spec for 9mm is 35,000 psi. For the .357 Sig, it is 40,000 psi. The additional flash and blast of the .357S round is not something I would look forward to from a 3.1" barrel Shield. The .357S is a high pressure round that benefits from barrel lengths around 5".
The list of manufacturer's making .357S guns is very short. The list of manufacturers not making them is long.
.357S is a niche cartridge with dedicated followers, but it is not a widely popular, money making cartridge for ammunition or handgun manufacturers. It has some advantages over other rounds, and some drawbacks. Apparently, the drawbacks outweigh the advantages in financial decisions of major manufacturers.
Standard 9mm expanding bullets are designed for velocities between about 1000 fps and 1250 fps. To function properly at velocities around 300 fps faster, 9mm bullets need to be tweaked for maximum performance. So, special bullets, special cases, special equipment to load the ammo, special guns to shoot the ammo, relatively low consumer demand = non-starter for major manufacturers.
For those who believe in the advantages, small aftermarket barrel makers stand ready to take your money to make your own .40-.357S conversions and beat yourself and your gun up with what amounts to +P++ 9mm ammo, but with reduced capacity at higher ammo costs with reduced loading choices and availability.
