357 Sig bbl

40 to 357 = ok...9mm to 357 = no go

Don't think you can convert a M9 to an M357 without purchasing an entirely new barrel and slide assembly.......which is more than a new gun on its own....probably be better off buying a new M357a1...if you can find one.

a 9mm barrell has a smaller OUTSIDE diamater as well, hence a .40 or .357 barrell would not fit through the slide.

I've checked.

now conversley a 9mm barell could be made w/ extra thick walls to match the diamater of a .40 to fit in a .40/.357 slide. I seem to remember there being one from IGB but did not see it on their list last time i went looking for it. While the slide is identical on the outside the difference is the diamter of the hole the barrel comes out (what is the gun jargon term for this anyway).

frames are completely interchangerable (except obviously a M or S, but even those you can swap the action subassemblies).

I know where there are a few 357 sig M"A1"'s

In addition to the diameter being different between a M9 and M357 or M40 slide, there are differences in the ejection port dimensions and cut outs within the slide that changes the weight of the slides.