The .357 Sig cartridge is basically a necked down .40 S&W case to accomodate a 9mm or .36 caliber bullet. Since it's practically the same in everyway as the .40 S&W cartridge it will fit in a magazine for that the .40 cartridge, and will fire in a firearm that is chambered in .40, but has had the .357 Sig barrel installed to replace the .40 barrel. I have no experience with the .40 S&W or the .357 Sig cartridge, but everything I've heard is the conversion works very well.
"It's a gun you loaded on Sunday, and shot all week."
"If God didn't create man equal then Sam Colt had made them equal."
"Fast is fine, but accuracy is everything."