My son just recently bought an M&P.40. He read or heard from somewhere that there is a 9mm barrel that will convert the .40 to a 9mm. Any advice on this is greatly appreciated.

If you can find one, just get an OEM 9mm barrel. Yes...it will work. I have shot countless 9mm rounds through my original .40 M&P. If you can't find the original S&W barrel, Storm Lake makes conversion barrels as well. There's will be better fitted, but the end result will be the same. The accuracy of my full size .40 with a S&W 9mm barrel is pretty outstanding and down to shooter error. I consistently will ring steel at 75yds+ while my buddies see dirt mounds with their pistols of choice. At 10 yards, the shots will be stacked.

Check MidwayUSA, they sometimes have the S&W barrels in stock. Or else check out Storm Lake barrels. Word to the wise, .40 mags will work but sometimes have feeding issues with the last couple of rounds due to the wider feed lips. You'll be best served to buy some 9mm specific magazines.

Midway has the OEM barrel in stock. Here is a link.
Smith & Wesson Barrel S&W M&P 9mm Luger 4.25

but having grown up with 1911s which are VERY sensitive to breach face fit and extractor tension and fit, how do you get away with it? The 9mm rim is nominally .032" smaller than the 40 S&W (.392" vs .424"). Needless to say, a bit of a sloppy fit. Does this have ANY impact on reliability or safety?
Keith

Proper conversion barrels have an offset chamber to bring the smaller round to the extractor. Ejection pattern may be erratic depending on the particular gun.

People running factory 9mm barrels in 40sw guns, idk, must work 'good enough'.

Honestly, mine makes the neatest pile of brass of any of my pistols at 3 o'clock.

Okay, I stand corrected, no one has any erratic ejection patterns because this guy doesn't.

My M&P 40 with the factory 9 barrel does have erratic ejection, but the accuracy is excellent with over 10,000 9mm rounds through it. I thought it was because of light 9mm loads with the heavier 40 slide, but even stout 1200 fps 124g loads causes brass to fling anywhere from nicely over my right shoulder to sometimes forward.

So you'd better take back your correction.

Did I say no one has ejection issues? I was speaking of my own experience. Guess I'll stick to providing my input on ARF and not provide any hands-on insight to s&wf. Sounds like purely conjecture is sufficient here.

Guess the rolling count now is 50% chance of erratic ejection patterns based on a whopping two data points!

(Sorry if the tongue in cheek snark was poorly relayed, thank you for your input etc etc)

Look to Storm lake of KKM for a proper 40 to 9 conversion barrel with the proper dimensions for tight lockup and reliable operation. I have fire more 9mm our of both my 40FS and 40c with Storm Lake conversion barrels than I have 40. Just picked up a second conversion barrel for the FS - this one threaded for suppressor use.

I'm running the factory barrel that I got from Midway USA. I've got around 500 rounds through it now and no problems at all. Ejection pattern is the same as with the .40 for me.

I have had mine with a factory 9 barrel for 2 years. No problems after several thousand rounds. I use the 40 mags with 9 ammo and no problems. It works great. Try it, and just get the factory 9 barrel for the conversion.

I used the Store Lake conversion barrel foe mine. It worked good except I had one batch of TULA brass ammo that were out of spec. and would seize up in the chamber and have to be knocked out with a wooden rod. All other stuff was fine.

One thought about the rim diameter: 9mm and 40 S&W use the same shell holder for reloading. Works well enough there.

A Storm Lake barrel will likely be a higher grade barrel than the factory one though.

The factory 9mm works fine, despite the naysayers. It's about half the price of an aftermarket barrel, too.

I'm with the get a 9 barrel and a couple 9 mags crowd. I have run a couple thousand rounds through our full size and both compacts. No issues. If you are thinking about doing this you should do it while barrels and mags are available. Depending on what happens with this election, supplies could dry up very quickly.

I know this is very late input, but just wanted to share my experience with making my .40 shoot 9mm. I purchased one of those Detroit PD M&P's in .40 back in 2017, and was fortunate enough to get one that had been unissued and was still in a sealed box with everything. It is a very nice handgun and I got it at an excellent price.

I regularly do firearms classes for new shooters, and allow them to fire a broad selection of guns, so that when they purchase a handgun, they have an idea of what works for them. This .40 was my only M&P at the time. But for logistical reasons, it was simply easier for me to only bring 9mm and .38 Special ammo to range sessions, and in my opinion, the .40 isn't a great "first experience" cartridge. I did have some 17 round 9mm magazines left over from when I briefly owned a previous M&P. I had had a negative experience with a Storm Lake conversion barrel (which to their credit, they replaced for free), with the earlier M&P Compact.

So I took a chance, and purchased a factory 9mm barrel for $62, and just dropped it in the full-sized gun. With the range ammo that I use, which is 124 grain NATO ball, it has worked without a malfunction, using 9mm magazines, for about 1,500 rounds thus far. Accuracy is on par with .40 factory ammo, but we typically don't shoot any further than 25 yards, and more often within 15 yards or closer, so we are not demanding very much accuracy-wise. I have not noticed erratic ejection, but to be honest, I haven't looked for it either.

In examining the factory 9mm barrel, and measuring it with a digital caliper, the only real dimensional difference is in what I call the "ears", the two little tabs at the top of the barrel hood that engage the breech. On the .40, they are .425" across, and on the 9mm, they are .400" across. So you do have about .025" of potential play at that point. I've seen no problems with this thus far, but then again, I'm not asking very much of the converted gun. I would not carry this gun when converted in this way as a 9mm, nor bet my life on it. But for occasionally shooting the gun with cheaper 9mm ammo, I think it is probably fine. A true conversion barrel is probably a better solution if one plans to do much 9mm shooting, or is striving for best accuracy.

I then was lucky enough to find an unused factory .357 SIG barrel for the M&P, and it of course works wonderfully, and I personally love the cartridge. So, my $300 police trade-in M&P .40 now can shoot three calibers in a pinch, for a total investment of about $450. In the current situation which has produced some ammo supply problems, this is useful.

Cheers!

I think you're passing up a great excuse to buy a new pistol. My MP40 Pro Series is a real nice, accurate pistol, why jack with it?

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A .357 Sig barrel will work in almost any .40 pistol. I bought a .357 Sig barrel for a friend who has a .40 pistol (but I don't remember which one, it's been a few years ago), no problem with fit or function. Except the .357 Sig caliber has relatively few friends. Never thought about using a 9mm barrel in a .40 pistol.