If I wanted a handgun that fired 9mm, .40 S&W & .357 Sig; do I start with a .40 S&W handgun and get the 9mm and .357 Sig barrels? Presume that I wou need 9mm magazines as well (?). Will .357 Sig ammo work in .40 S&W magazines?

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otis

In my experience, yes to all

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“Look life in its iron face”

I'd think starting with the 357 SIG would be the way to go. Replacement/aftermarket barrels are probably more plentiful in 9MM and 40 than the 357sig.

Glock 26/27/33 trifecta would be one way to go.

9mm in a 40 mag is probably iffy. It's probably going to vary from one gun/mag combo to another. Certainly not ideal for serious business.

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Just Say No - To Social Media

Get a GLOCK 22. You can shoot all three cartridges. The G22 is a .40 S&W and the magazines work fine with .357 SIG. A simple barrel swap for .357 SIG and for 9mm, a barrel swap and G17 mags work.

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GOA FL Dir. & Nat. Spokesman

Barrels are tough to come by right now. Most OEMs have long lead times. Some pistols require a little more tuning to step down to 9mm. My Polymer 80 G35 wannabe is sprung heavy to shoot softly and have a lighter trigger pull, it's on the edge for .357SIG and I doubt it would reliably cycle with 9mm...but I have a G34 to do that so I don't worry about it much.

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Psalm 27:2

During previous 2010 panic I bought a used S&W M&P .40 S&W. Bought 2 barrels; one in 9mm and one in .357 Sig. All work well.

If you're going to go that route, then I suggest starting with a Glock or a SIGsauer chambered in .357 SIG.

Why? Because Glock and SIG are the only two companies that I know of who still make guns chambered in .357 SIG, and seeing as .357 SIG is substantially higher in pressure than 9mm Luger or .40 S&W, it seems best to start with a firearm that you know can handle .357 SIG pressure/recoil because it was chambered for it.

A lot of folks suggest starting with a .40 S&W then converting it to .357 SIG, but .40 S&W is 35,000psi whereas .357 SIG is 40,000psi, ergo a .40cal pistol isn't designed for .357 SIG pressures, and while that can be overcome by swapping out springs everywhere, it just seems like a simpler, easier process to buy a pistol chambered for .357 SIG then work your way down to 9mm Luger/.40 S&W than the other way around.

Also, as someone who has repeatedly explored the concept of performing .357 SIG conversations on .40cal pistols, I can personally attest to just how hard it is to find .357 SIG Conversion Kits, especially for anything besides a Glock or SIG brand pistol.

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Shooting Comfort is bilateral.

What, no love for the 41AE?

357 Sig ammo works fine in 40 S&W magazines.

So far as the rest is concerned, I'll defer to others on the advisability and which should be had first. I found that when it came right down to it, whatever the gun (Browning HP, HK P2000SK, S&W M&P Compact, EAA Witness, Ruger P89, etc) I fired the more expensive round only rarely and not as much as I expected. May be my "hate to lose brass" re 357 Sig, 30 Luger issues.

Just throwing this in as a wild idea... seems like you could add 10mm into the mix, by starting with a 10mm and finding a way to add magazine spacers like the early 3rd gen 40’s had?

Sig P239...the .40 version will take .357Sig and 9mm barrels, and feed em
with the .40 mag.

Got a .40 239 police return a couple of years ago. CDNN still has a
slew of (threaded!) new Sig brand, 9mm P239 barrels for $90. Drop
barrel in and go, all there is to it.