I'm pretty sure it's a typo. The .357sig barrel is a drop in conversion from .40sw, but not 9mm. There are conversion barrels (most of them made by EFK) that will go from .40sw or .357sig to 9mm, but never the other way around. Even with the .40/.357 to 9mm conversion, you may experience reliability issues - especially FTEs due to the larger breech face...so what do they mean by this "This barrel is a direct replacement barrel for all HK USPC .40 or .357 and can be used to convert 9mm pistols. This is a great spare or replacement barrel. Also, a must if you want to convert your to .40 USPC or 9mm to a .357 Sig."
I assume they mean machine work is required for the 9mm to be converted to .357sig?