I wrote the following on AR15.com-
I ordered a factory M&P 9mm barrel for the fullsize from Brownell's this week. It came today. My plan was to see if it was dimnsionally the same as the 40 S&W, since the 9 and 40 M&P fullsizes are, essentially the same guns.

I took some measurements and some side by side photos-











The 9mm barrel locked up nice and tight, It was not loose or ill fitting in any way. I had called S&W and asked a tech guy about the dimensions of the 9mm and 40 slides. He said that dimensionally they are the same. Of course he went on to tell me that I should buy a dedicated 9mm and not use a different barrel...then he said quietly, wink, wink.

Then I broke out the digital calipers-

40 S&W barrel-




9mm barrel-




Next was a function test. Using the new barrel, my M&P40, some Tula 9mm from Wal-Mart and a 40 S&W mag-I have a 9mm mag on order from 44mag.com, it's just not here yet-I went to test it in my unofficial test range out back.

It functioned flawlessly, with one exception-right before the last round, the follower would engage the slide stop, not feeding the last round. That is due to the mags being for the larger diameter 40 round. The smaller 9mm round allowed the notch in the follower to hit the slide lock before chambering the last round.

I expect that to be solved with a factory 9mm mag. I would have had a magazine had any of the local shops had one in stock yesterday, when I was in town.

I will test it again when the new mag arrives.

Here is the schematic to the M&P from Brownell's-
Schematic for Smith & Wesson® MP (Semi-Auto) - World's Largest Supplier of Firearm Accessories, Gun Parts and Gunsmithing Tools

As you can see, the 9/40/357 guns share the common parts, with the exception of the barrels. 40/357 has it's mag and 9mm has it's mag.

UPDATE 101/03/2011-

Went to the range today for a test. I traveled to Louisville, KY to shoot at the an door range-Welcome to Bluegrass Indoor Range located at 4402 Kiln Court Louisville, KY 40218 (502) 451-9088. Great bunch of guys there and the prices are affordable.

I took 200 rounds of 115gr 9mm. 150 of it was Tula, not know for being all that clean and 50 rounds of Federal Champion.



Distance was 21 feet. Course of fire was slow, double taps and rapid-as fast as the range rules allowed.

No failures to feed or eject, one failure to fire and I blame that on a hard primer-it was the Tula ammo. There was a noticeable dent from the firing pin. I reset the sear and it did fire the second time.

The Tula was nasty..very dirty. The Federal Champion had a huge fireball. The Tula did not.

All and all, I'm happy with the test and now have 2 guns in one.