40 to 9 conversion barrel?

I am toying with the idea of converting my m&p 2.0 5” chambered in 40 to being chambered in 9mm. Any thoughts or opinions? Would you use a gun that has been converted for self defense?

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I went the other way and converted .40 to .357 sig. The vast majority convert (glocks anyway) have no problems. My gun only needs a barrel swap to be a factory sold .357 sig anyways. If your research of your gun looks promising, go for it.put a few hundred rounds down range and see how it works. I like mine and am happy.
Edit…mine is also a storm lake barrel.


@Scoutbob and @Brian1

I was wondering a few things about this as well. I’m considering both a 9 and 357sig KKM conversion barrel for my 40 M2.0. Has anyone compared the 9mm conversion with a 9mm RSA vs the 40 RSA?

The P series SIGs were built to do this with specific limitations. Even had a .22LR conversion kit with slide, barrel, magazine, & spring set for a P220. And having a rig originally built for higher pressure, more kinetic punishment (40 or 357) I’d say going to 9 should work though you may need a softer cyclic spring to properly operate on 9s. Good Luck. Having the options makes the pistols even better than they were.

Personally, No, if it’s my self defense EDC gun. I will not change a single thing on it.

Reasoning, it takes away one less outlier for a prosecutor to use against me if, God forbid, I ever have to use it.

Competition gun, sure. Range gun, same thing sure. But I place those and their ammo in a different safe than my EDC.

That’s why I have multiple Glocks. They ain’t sexy, but they are reliable.

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I converted a Glock 23 to 9mm, shot it much better. Kinda miss that gun…

My next 1911 is going to be a switch caliber, 9mm and .38Superb.

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I would be reluctant to try this conversion using just a swap barrel.

Boltface diameters differ between 40 S&W/357 SIG (.425") and 9 x 19/7.65 x 21 (.391"). Accordingly, extractor engagements and functional integrity can be compromised. A better plan might be to assemble a new slide with proper extractor settings–proper recoil spring tension–correct barrel–and possibly proper magazines for the cartridge involved.

I have a Ruger P-89X that came from the factory with 2 barrels–one in 9 x 19, the other in 7.65 x 21 (30 Luger). It also came with 2 recoil springs–one for each caliber and their differing backthrust rates. Both cartridges feed well from the OEM magazines.

The 357 SIG and 40 S&W perform with no changes in magazines or recoil springs between the calibers.

I suspect that outfitting an existing 40 S&W pistol to properly and reliably fire 9mm ammunition will wind up costing almost as much as a new 9mm pistol of the same make/model.


I’ve decided to keep it chambered in 40. I actually really like 40 S&W. It’s just expensive to shoot :sob:.

The agencies won’t cop to it–but I suspect that cost factor is driving a lot of law shops toward the 9mm and away from the 40 S&W. Of my old shop’s 3 primary autopistol calibers (9mm, 40 S&W, 45 ACP) the 40 S&W is viewed as the most difficult to manage due to snappiness of recoil impulse. Cop shops use between 18-50 rounds for one qualification session, and a failure-to-qual brought on by sharp recoil means a re-shoot, with 18-50 MORE rounds getting consumed. Project that tendency over an agency the size of LAPD or LASD, and the cost factor gets huge in a hurry. 9mm is easier to shoot, esp. in its usual USA-level loadings that run 15%-25% less powerful than European 9 x 19 specs. In its homelands, the 9 x 19 runs 123-125 grain bullets at 1225-1250 FPS. That is about the mid-point between 38 Special +P and 357 Magnum. Most USA-loaded 9mm ammo is about a 5% uptick from 38 Special +P. Euro-level 9mm is a very different animal than our domestic milquetoast loadings. This is why I DO carry 40 S&W and 45 ACP, and not the 9mm.

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I really like 40. The ballistics are good and you can get more ammo capacity than 45. Its a snappy round, but it’s not bad at all in a full size gun. This pistol is primarily home defense. I’ve been moving twards the larger calibers.