Barrels need to be make specific, the geometry of a Sig barrel is going to be different than a Smith barrel.
I just bought a Used Sig Sauer P229 .40 cal. Great condition. I have a S&W M&P .357 Sig as well. I know that when I bought the M&P they said I could convert it to a .40 with a barrel change. Does it have to be a S&W barrel or can I use the barrel from my P229? I am pretty sure they are going to be manufacturer specific but it is worth putting the question out there!