The Sig .40 S&W to 9mm conversion takes more than just a barrel (unlike the .40 S&W to .357 Sig conversion). The kits that Sig sells (slide, barrel, and magazine) work quite well but since they are Sigs they are pricey. T
My 226 runs 9mm, .357 Sig, .40 S&W, and 22lr. I think the conversion kits are worth it and are as reliable as the original parts.