I have an M&P40c and really like it. I learned on here that I can get a stock 9c barrel and some 9mm mags and shoot 9mm at the range. I've done that, it does work and it's great. Thank you. But this leads to another question.

I'm a tinkerer and have worked on motorcycles, built guitar amps, etc. Most things I've worked on have schematics and you can see the specific part you need and you can see how something that looks like a similar model, has differences. I haven't been able to find any "schematics" for these pistols. I'm curious to know what other differences there are. Is it really just the barrel?

So here I am to ask you. What other differences are there between the 40c and the 9c? And can these differences affect reliability? Swapping the barrel at the range is great, and it's not a big deal if it has the occasional malfunction (which it hasn't in 9mm mode). But could one swap to 9mm and be confident carrying it for self-defense? If not, why? What parts are different?

I'm not looking to carry it in 9mm for self-defense as much as I'm looking to learn. Also, it's good to know what options and limitations I truly have.

Thanks for your time.

I have the same setup. Other than the extractor and a slightly larger boltface in the .40, I don’t believe there is much difference, if any.

I read that the conversion will only work one way. That is you can drop a 9mm bareel into the stock 40 caliber gun/slide. But you can NOT drop a 40 bareel into a stock 9mm gun.

The slide is different in some milling for the 40 at or near the enection port, that the 9mm slide does not have. I also think the 40 slide is a bit heavier and has more metal to help with recoil. But could be wrong about that.

With the slight offset of the extractor on the 40, I need to shoot warm/hot 9mm to get the cases to eject properly. With 120g bullets going any slower than 1100 fps, the cases often hit my forehead or land on top of my head.

What brand of 9mm barrel did you guys go with? And I assume normal S&W 9mm mags?

There are many differences between the 9 and the 40. If you have a 40 switching to 9mm with barrel and mags is fine for training and cost savings. If you looking to convert for carry buy a 9mm gun. The cost of replacing the complete slide assembly and possibly the ejector to 9mm is too costly. Just go buy a 9mm. The differences are the breach face is bigger on 40, extractor, ejector angle, barrel, recoil spring assembly, magazines are all different. To replace these parts cost as much as a new 9 mm compact. The best conversion would be to 357 sig hands down. Just a hotter 9 but more expensive to run.

I suggest you spend some time on MidwayUSA, Brownells, Speed Shooters Specialties and a few other sites that sell M&P parts. You'll find many of the parts are the same. In fact many parts also fit the compacts and full size.

I purchased a S&W factory barrel from Midway, not issues what so ever with, using factory magazines because I don't trust aftermarket. No problems using it at the range.

The difference is in the breech face and hood dimensions.

This is a factory .40S&W slide with factory barrel:



This is a factory .40S&W slide with factory 9mm barrel:



The 9mm will fit, but might have some accuracy issues. I doubt it will have reliability issues.

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To the best of my knowledge this is the correct answer. Aftermarket conversion barrel makers take the .40cal hood dimensions into account and mill their breeches to fit snugly in the .40's slide improving the odds of keeping accuracy up to par over a sloppy fitting factory 9mm barrel.

I opted for a StormLake conversion barrel for my personal full sized M&P40 and use factory 9mm magazines. I have been very pleased with the accuracy and reliability of this set up.

I would compare it to going from shooting .357 magnums to .38 specials on a revolver. Great way to get in more cost effective practice using the same platform while reducing the physical punishment on the hands and wrist.

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Good info. Thanks.