I have a full size M&P 357 Sig that I want to convert to 40 S&W to take advantage of cheaper ammo. Can I just drop in a 40 barrel or do I need to change the recoil spring weight as well?

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NRA Endowment Member

from what I just looked up, 9, 357 and 40 all use the same 16# spring. You could also get a 9mm conversion barrel. The bore is .0225" offset to the right so you get proper extractor hook engagement.

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NRA RSO

Those 16lb springs were from the early years of the 1.0 series I was told they were changed to a 17lber some later after dumpng the 357sig from the line up !? I was told by s&w CS the 2.0 40sw runs a 18lb springs . I also found this spring runs well in my wifes old 4.25" full size core 9mm for many years . I run a 20lb spring in my old 4.25" full size m&p 40sw and that runs fine with standard 40sw loads and with 155gr underwood HP ammo . I also have a 5" 40sw thats running fine with a federal factory or my reload ammo with the 18lb factory green spring .

By the way a glock 17 gen 1 thru 3 springs are the same length and work fine in the m&p .

I've had two different companies - Sig Sauer and Rock Island - tell me that their 357/40 pistols use the same recoil spring.

I got sucked into the modular caliber thing with Sig Sauer years ago... they use the same spring for 40S&W and 357SIG..

I went the other way. I converted my 1.0 M&P 40c to .357 SIG. I did not change the recoil spring. I have since shot nearly 3,000 rounds of .357 SIG with no problems, so I expect that your .40 will do just fine with the original .357 spring.

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What, me worry?

Works great in the both fullsize & Compact M&Ps (& with 9mm as well) in my experience...

Also in the FN FTX, for what that's worth.

Cheers!

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Sit Reloader Sentiret

I went the same way. I have an M&P 40, a 40 Compact and a Sig P229 that I have done the barrel swap on plus a G32. You can't beat the accuracy of the 357 Sig and it is a pretty stout round as well. I cast my own bullets and PC them. I don't push them quite as hard as the jacketed bullets but it is still an attention getter at the range when people think you pulled out a 9 or a 40.

You wouldn't consider selling that would you? I have the compact version and I've been kicking myself for years for not getting the FS at the same time when I had the chance.

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I recently researched this and learned there were 2 different full size recoil springs, green and blue. My understanding is that greens were fazed out and all calibers got blues.

yes, I have both a P226 and 2 P229 40 and 357 sig use the same spring.

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NRA RSO

FWIW, my P229 uses the same spring for the 40 and 357 and works flawlessly with both cartridges. Have no idea how many pounds it is.

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213th FBINA

Most seasoned shooter would not feel the difference in a spring thats slightly heavier or lighter but if your good at placing a number of rounds of several targets quickly you will see the difference with a shot timer and it all depends on the load you shooting . Buy a few different spring and try them and make up your own mind .

One of my two carry guns is an m&p 4.25" 40sw filled with 155gr underwood load that averages 1318fps with a 20lb recoil spring . But its tested and reliable with both hst 180gr and ranger t series 180gr standard pressure ammo but I can see it go nose down when chambering the next round so 18lb spring would be better for that load . My typical practice load is a 155gr at 1280 fps and the 20lb springs feels about right and it shows with a shot timer too .

Figure out what works best for you and the load you shooting . If you see your pistol when going back in to batter and go nose down your recoil spring should be lighter . When my wife was shooting local uspsa with 147gr 890fps minor PL load in her m&p core the recoil spring will be a 13lb spring but when I used it at major PL an 18lb spring worked well .

Figure out what works best for you !!

The .40 cal recoil springs in my two M&P Compact 2.0's (3.5" and 4.0") function equally well whether I shoot .40cal rounds or .357 SIG rounds. Drop in the barrel I want, load up the (same) mag with proper rounds, pull the trigger. Goes bang every time with no FTF issues at all.

Just exchange the barrel and go shooting.