I read somewhere that you can convert an M&P 40c to a 9mm by using a Storm Lake 40SW TO conversion barrel kit.
BARRELS SLBSWMPC9C358
STORM LAKE BARRELS S&W M&P COMPACT 40SW TO 9MM CONVERSION BARREL KIT - SLBSWMPC9C358

Is there any trick to this? or is it just a matter of inserting the 9mm barrel?

My main reason is to save money at the range. But continue to use the 40 cal for carry.

Can be done-also have to buy the 9mm mags though!!!

Other than getting the 9mm mags, is there nothing more to do than put the new barrel in the frame?

While the conversion barrel is designed as a drop-in, there is no absolute guarantee that it will. My gunsmith tells me there have been a few that needed a touch of fitting. If you have any problems, you can discuss them with Storm Lake before taking it to a gunsmith. If you don't have any problems, then happy shooting!

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Science plus Art

"you can discuss them with Storm Lake before taking it to a gunsmith"

For an advertised drop-in barrel, I sent my SL 9 mm barrel back twice to them before I could even chamber a bullet. My take is that you shouldn't have to take it to a gunsmith, particularly given how popular this particular barrel is, it should be made right. They paid for shipping, but it took at least 2 months before I had a working barrel. With the cost of ammo, I am now a happy camper at the range. It fires all day with no problems. I bought the FS 9 mm mags with X-grips, so I have less time loading.

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Recovering Pacifist

I just purchased a complete top end with a few mags. Works fine for me. I was going from a 9c to a 40c. 40 & 357 Sig can be done with standard barrel changes and the mags are the same. 40 to 9 has to have a conversion barrel or different top end.

I use a factory S&W 9mm Compact barrel in my 40c with 9mm magazines. While I would not depend on it for defensive purposes, I have probably put close to 1K practice rounds through it without any problems.

I measured the barrels against each other with digital calipers and they were within a few thousandths in all critical dimensions.

There is a good write up if you search for it where the guy compares two factory barrels and they are so close that many just drop in a factory 9mm for $65. I have heard about more problems using the Storm conversions than a factory 9mm barrel, but maybe more people try the Storm barrel, I don't know.

I have had no issues with my factory 9mm barrel

I have a 357c and use the 40c factory barrel in it. I also have a factory 9c barrel I use. There has been no problems for me thus far. Of course you have to get magazine for the 9mm. I will carry any of them as my defense rounds. However, that being said I mainly carry 357 just because I like the round for my needs and wants. Would I feel scared carrying my 9mm barrel in my 357 not at all. Plus the factory barrels are cheaper and drop in.

I have no experience with the C, but my MP40 runs 9mm great with just a factory 9mm barrel and mag installed. 100% reliable with 115 and 124 FMJ so far (I have not shot any JHP through it because I just intend this as a cheap practice setup).

What about the recoil spring. Doesn't the spring for the 40c expect more recoil force to cycle properly?

Nope. the .40c and the 9c use the same recoil spring.
Yellow end.

When you say "top end" do you mean the slide assy., with barrel and spring?

OP, you might give S&W a call and see if you can buy a factory barrel from them. Depending on who you talk to you might get lucky as I did a few months ago.