From what I've read on this website, the standard 9mm barrel from S&W is NOT recommended for this conversion. S&W recommends a conversion barrel along with the 9mm mags. To my knowledge the only standard barrel you can drop in a 40 to change calibers is the 357 Sig.My understanding is that you need the 9mm barrel and magazines, everything else is the same.
+1 :yes I'm sensing a pattern Mark!I can tell you, being a pistol smith for over 4 years with the M&P series, you are flirting with disaster using a 9mm barrel in a 40 cal slide. There is a .20" difference in the chamber hood between the 9 & the 40 barrel. This creates an incomplete lock-up of the barrel that has caused catastrophic failures. Would you run 14" tires on your 15" rims, just because they fit and other people do it & haven't had any problems ? ... YET! Find the post from KRWeiss about his friend's disaster using a 9 barrel in a 40 slide. Do yourself a BIG favor.. spring the extra money for a conversion barrel.