No, you would need the Sig P226 .40 to convert down to 9mm barrel(you would also need a 9mm magazine). Note: This barrel can be dropped into a Full Size SIG Sauer P226 chambered in .357 SIG in order to allow you to convert from .357 SIG to .40 S&W. sigsauer.com/store/parts/conversion-kits.html

Mike, you'll need the entire conversion kit to move from 9mm to .40S&W. It's not as easy as swapping out barrels. That barrel is designed to be dropped into a .357 model P226.
sigsauer.com/store/parts/conversion-kits.html?model=121