Conversion barrels

I have a S&W .40 Sheild. I love the .40 as a carry round and the M&P line. But after doing some research I like .357 sig more.
Only problem is they dont make .357 sig and I’m going to transition to that for my carry caliber.
I contacted S&W and they dont recommend it but I realize that could be purely for legal reasons.
Anyone out there know the real scoop if I could safely put a .357 conversion barrel in S&W .40 handguns weather it be the sheild or others in there M&P line assuming of course I get a quality conversion barrel and use quality ammo.


I’m no expert, but it seems there’s a lot more to it than swapping out a barrel. What about the magazines? Extractor?


I brought this up playing with the idea of switching from 40 to 9mm.

@OldGnome is right. It’s a bigger process than just swapping the barrel.

40 to 9 is a more simple switch because many 40 guns share the same dimensions with 9mm. There’s no need for a new slide. Might need new recoil spring, extractor and there may be some internal work recommended.

I love the shield, but there is not a .357 model. This means if there is a mag, it will be an after market one. Same with barrel. Same with spring. Same with extractor. You want a carry gun that you can confidently know it will go bang. You also want the gun rated and thoroughly tested to handle the round you’re firing.

It’s always best to buy the gun chambered in the round you want.

357 Sig is a necked down 40 S&W. They use the same magazine. I have not done it, but my research indicates a barrel swap is usually all that is required.


It’s been a while since I was considering the same swap, so just looked at muzzle energy. The 357 Sig seems to average about 100 ft-lbs over the 40 S&W so might want a stronger recoil spring to smooth out recoil and reduce slide to frame impact.

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Check this out. There are similar threads and discussions all over if you search and reamers can be had at Brownells and several other places

Reaming 9mm barrel for 357 sig

From one of the other threads:
“Read the OP again, folks. The guy has a Ruger chambered in .40S&W. He rechamberd a 9mm barrel and put it in his .40S&W pistol. That would work.”