.357 = .40 case necked to 9mm
9mm slug and .40 rim. With a .357 Sig chambered barrel you can reliably change from a .40 S&W to a more expensive .357 SIG. With the 9mm barrel as others have said the rim on the casing is smaller and the extractor will have some issues snagging that rim. (you might need a lighter spring, too)