I am looking to buy a 9mm barrel for my MP .40. I was told that if you were to buy a SL barrel that the regular 9mm will work just as good as the .40 to 9mm conversion barrel. Is this true? Does anybody know the SL part # I will need?
Will a S&W 4.25" factory MP barrel work in the 4.25" MP.40?

In general the 9mm barrel will fit the 40. The reverse is not true. I have Storm lake conversion barrels for my Glock 23 and they are very good. For the price though, you could buy a new pistol for less than $75 or so more.

You can buy an M&P FOR $225? WHERE!?!?!

A factory barrel will work, but the hood is lightly narrower than the 40, so there will be a little play side to side in the slide.
Personally if a conversion is available, I would go that route, as its made to fit in the 40 slide.
OpticsPlanet has the conversion for $124.79.
They also have the factory 9 for $129.49.
Shipping is free. Also use code FATHER and get 10% off till 6/19

StormLake 34131 SW-MP-9MMC-425 M&P Full-Size 9mm Conv For 40S&W/357Sig 4.3" SS 27% OFF

StormLake 34126 SW-MP-9MM-425 S&W M&P Full-Size 9mm 4.25" Stainless Steel 23% OFF

My stormlake 40 to 9mm conversion barrel works flawlessly with any 9mm ammo in my mp40c.

Additionally, all my conversion barrels from lonewolf and stormlake run perfect, never had a malfunction with glocks (all gen 3 and 4 models) or MPs. I trust my conversion barrels.

I've not had great luck with the conversion barrels I've had. I picked up an apex semi drop in barre for my 40 3 weeks ago. So far with about 550 rounds through it with 5 fte and 1stovepipe.

The storm lake I had a few years ago would fte/ stove pipes at about a 5% rate.

If I were you, I'd get the oem 9mm barrel (assuming it's less than $90 or so) and if it worked well, rock on. Between the SL and the Apex, I'd get the Apex hands down. The accuracy is great, noticeably better than SL.


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Have Storm lake barrels in 9mm and .357 Sig for my FS40. Both shoot great, ZERO malfunctions. The 9mm would likely hit to point of aim as the factory .40 barrel did with 180gr in mine, but I found that .357 Sig and 165gr .40 shot a bit low, so I replaced the front sight with a shorter one and both .357 and .40 in 165gr hit to point of aim with 115gr 9mm hitting a couple inches high.

I went and got an OEM 9mm bbl off Ebay. It also runs fine.
I guess I've been lucky so far because the 9mm conversion
barrels on everything I have run
well and shoot better than I do.

Another source is Remsport:

.40 to 9mm M&P Shield Conversion Barrel

__________________
never get out of the boat

You bought a 4.25" S&W OEM 9mm barrel and dropped it into your FS MP .40, and it worked ok?

I did that about 5 years ago with my M&P40 FS. I also got 9mm magazines and have put about 10k rounds through the combination compared to about 45k of 40 through the same gun. My experience is that with weaker9mm loads, the empty cases sometimes land on top of my head, but with 120g bullets at 1100 fps or greater, the cases eject nice and strongly off over my right shoulder.

WOW! And to think I spent extra money on a .40 to 9mm SL conversion barrel!