Can a P226 40S&W be converted to 9mm by just changing barrels and mags ? Is a conversion barrel needed or can a standard P226 9mm barrel be used ?

While there are a few posts here indicating success with just using an OEM native 9mm barrel with the P226, I would be inclined to go with a conversion barrel. Some .40 mags/pistols may function OK. some may not. For SD, I'd go with conversion barrel and 9mm mags. Regardless, test the hell out of it before carrying.

I'm not opposed to using the native OEM 9mm barrel on P239 .40's. Mine functions 100% with both .40 and 9 mm mags. Many others have reported the same.

As stated above, get the conversion barrel. You can do 9 mm and/or 357 sig. Use the appropriate mags and enjoy.

I have done so but the mags do not usually lock after the last round.

Would someone put a caliper on a SIG P226 OEM .40 barrel and a SIG P226 OEM 9 barrel? Please? If an OEM 9mm barrel would fit in a .40 P226 slide, why would SIG sell a 9mm conversion kit that includes a slide?

The barrel diameter is the same, but the barrel hood is narrower on the 9mm barrel and doesn't lock into the breech block as well as the .40 barrel.

The breech face is wider on a .40 slide than on a 9mm slide which is why Sig sells conversion kits that consist of a barrel and slide.

The breech face is 9mm sized on a 9mm, and 10mm sized on a .40.

Thank you, gentlemen.

Or you could do what I did- source a complete Sig 9mm upper and never worry if there will be fit men issues.