S&W CS says M&P 40 to 357 sig conversion unsafe.

[Walt Sherrill]

Typically, the S&W (and SIG and GLOCK) .357 SIG models share all components with the .40 models EXCEPT a barrel. There's generally nothing else different. Even the mags are marked ".357/.40". With some brands, the caliber is marked on the slide; with others its on the barrel, so that in some cases you may NOT be able to tell that a different barrel has been installed.

A true conversion barrel lets you use an after-market barrel to change the gun's caliber (from .40 or .357 to 9mm, for example.) The conversion barrel is adjusted so that the extractor works properly and headspace is correct. New (9mm) mags may be required, but that's not always the case. Some Glock shooters buy after-market barrel to get away from the Glock polygonal barrels -- The after-market lands/groove barrles lets them shoot lead bullets. Changing barrels is not a big deal, and quite common.

You may feel more confident shooting a "factory condition firearm", but the gun won't know the difference and the target won't, either.

 

[Flash_80]

Guess I'm just living on the edge

 

[PPS1980]

As Walt said, the hyteria over droping a S&W 357 barrel into a slide that is marked 40 is well.....hysteria. The frame, slide, etc. are the same for the 40 & 357 and as he said the S&W magazines are marked 40 S&W/.357. The only difference in those two weapons is the barrel. Schematics of the M&P indicate that the parts are interchangeable across the two .40 and .357.

 

[lgjhn]

As PPS1980 said: Same gun, different barrels. I'm a fan of the .357SIG and haven't had any issues using it in my M&P40. As he stated, even my magazines are stamped .40S&W/.357. Yeah, ammo is expensive. Reloading is a pain, but can be done. I don't find the recoil much worse than the .40. It's more of a push back into the hand rather than the flip that I experience shooting the same gun with .40. It's certainly not the caliber I'm gonna take to the range and shoot 300 rounds for factory ammo, that's for darn sure, but for defense, it's a pretty devastating round. I reload it to slightly milder loads for range and reliability work. To each his own, I guess.

 

[gojones]

I have a Sig P226 and shoot both 40 and 357sig. It is a beefier frame and internals and have had no issues. I would not, however convert my M&P to 357sig. With the Sig, it is simply a drop in barrel from 40 to 357sig. Going from 40 to 9 requires other replacements, recoil spring and etc.

 

[Walt Sherrill]

Going from 40 to 9 requires other replacements, recoil spring and etc.

That's the theory, but that's not always true when you actually do it.

With most guns, the only difference between the .40 models and the .357 SIG models is the barrel. And you'll find that the only major difference between .40/.357 SIG and 9mm models is the slide, which is often a bit heftier with the .40/.357 SIG versions, which helps control slide velocity. (It may have changed with Gen 4, I don't have one of those, but with Gen 3 Glocks, the recoil springs are the same for guns that share a frame size.) There were and are exceptions:

I've been told that when SIG created the P229 to use the .357 SIG round, they made the slide heavier than in the 228 model, which had been their starting point. A heavier recoil spring alone did the job, but made it very difficult to rack the slide. They also beefed up the frame abutment/locking blocks for the barrel. In THAT case, it was more than just a heavier slide. They have also made their .40 models a bit stronger than their 9mm models.

When FNH created a .40 version of the Hi-Power they had problems, but later changed to a CAST FRAME (rather than a forged one). That solved the problem, but the demand for a .40 BHP was never all that great. I'm not sure anyone ever offered a .357 SIG barrel for that gun. FNH offered a two-slide verison of the FNS-9/40, and the only differences were the slides/barrels.

Berreta at one time offered a 9mm/.40 combo and again, the only differences were the slides an barrel. I'm not sure about the recoil springs.​

I've had several SIG .40/.357 SIG guns that ONLY required a conversion barrel -- and in one case a magazine. With a SIG 2340 (.357 SIG), when running a 9mm conversion, it wouldn't lock the slide back with the last round, whether I was using a 9mm mag or a .40/357 SIG mag. SIG said the slide stop was the same for either model when I called them. I lived with it.

With small-frame metal-framed CZ-pattern Witness guns, you didn't even have to use a conversion barrel, as the factory 9mm and .40 barrels had the same outside diameters and were interchangeable. Mags would work either way, but 9mm mags worked best when shooting 9mm rounds -- at they positioned the top round a bit higher as it hit the ramp.

Hickock45, on YouTube has shown that you can get surprisingly good results by just changing factory barrels in many Glock models, even though the barrels don't match the slide opening. He could put a factory 9mm barrel in a .40 or 357 slide and hit the distant targets without problems. Nothing else was changed, or needed. (He may have changed magazines, but I don't remember him mentioning it.)

I've had similar experiences with conversion barrels in Glocks (One of my Glocks, a 23, came to me with .40, 357 SIG and 9mm barrels, and I generally didn't always have to change mags. I didn't change recoil springs, although I had them.

With my FNS-40 9mm conversion, hardball feeds well, but the type of hollow points I use doesn't -- so I picked up a couple of 9mm mags. I didn't change recoil springs, but had them.

Conversion-barrel makers build their barrels so that headspace and extractors still work without changes. I haven't heard of ejectors having to be changed, but that might be required with some gun brands.
.

 

[PPS1980]

I ran my MP40 today for the first time. Great out of the gate. Fired 45 FMJ in 40 no problems. Then swapped out to 357Sig and ran 45 FMJ, again - no problems. Easy and all functions were as expected. No FTF or FTE or any other issues at all. Got a good bit of interest as I was swapping barrels. Very happy with my new (to me) M&P 40/357.

 

[Disabled1]

I ordered my .357 SIG barrel from Midway this morning. I also ordered 100 rounds of .357 SIG ammo from SGAmmo.com

Smith & Wesson Barrel S&W M&P 357 Sig 4-1/4

357 SIG Pistol Ammunition | SGAmmo.com

 

[Flash_80]

Still waiting on SG to get that Speer Lawman back in stock for $16 a box. Thats hard to beat.

 

[Blww]

I called s&w asked if 40 and 9 were the same lower

The woman said they were not allowed to gunsmith or recommend but they were the same but added I cannot tell you that. My question is are 40 and the 357 sig they use to make well in gen1 the same lower I know the 45 is not the same as the 40 or 9

 

[swsig]

This is an old thread, but since it has been revived, I'll relate my recent experience with my 1.0 M&P 40 compact .357 SIG conversion.

S&W used to make a .357 SIG 1.0 M&P until they didn't. Aftermarket manufacturers stepped up allowing .40 S&W M&P pistols to be converted to .357 SIG. After shooting 13,000 rounds of .40 S&W through my M&P 40 compact, last year I converted it to a .357 SIG, using a Storm Lake barrel. I have over 2,500 .357 SIG rounds through it now, with no issues. The 1.0 M&P 40 chassis appears to be stout enough to handle the .357 SIG round.

For a full version, see my long term report here:
Long-Term Report on M&P 40c, Shield 9, & Shield 45

 

[colt_saa]

My question is are 40 and the 357 sig they use to make well in gen1 the same lower I know the 45 is not the same as the 40 or 9

The 357 & 40 chamberings share a frame

My oldest M&P is over a decade by now.

Nothing but full power 357SIG ammunition through it for those 10+ years

Hey worst case is you use it so much that you wear out the gun and need buy another, just like we do with our cars

 

[hardluk1]

LE was known to have issues with the 357sig enough for s&w to stop production of them years ago . Now maybe some had tighter tolerances like maybe the Wilson county model shown above But I bit I shoot more rounds in a year than most LE do in 10 years .

I acquired a M&P 4.25 40 a while back and change the recoil spring from the old stock 16lb spring to a 20lb recoil spring like the 2.0 use . and that change alone made for a better shooter Glock spring are the same and 4.25 and 5 " models are the same for m&p . See how this effects accuracy before and after shooting from a sand bag . Then how well your pistol may hold up as a 357sig may also depend on well the barrel fits too .

I bought a apex gunsmith fit 9mm barrel for my wifes M&P PS CORE 9mm as it shoot group more like a shot gun than a pistol at 25 yards . The gun smith fit barrel needed only 2.5ths off the hood to fit tight but still run reliably and turned that pistol into a some far closer to a bullseye pistol than a defensive pistol . Note that Nothing needed to be removed from the lug area , it was darn near a drop in barrel . Sloppy s&w production tolerances make or break these early 1.0 models as they had QC issues .

 

[HOUSTON RICK]

I am not a tinkerer, because I am not good at it. Some guys can customize and re-chamber their guns with great outcomes. I am not there yet. UNLESS you know what you are doing, best to shoot rounds in a gun that was factory built for that round. Not trying to warn or aggravate the many here who do know what they are doing.

 

[hardluk1]

rick Sounds advise for your self since your not there yet . Guys have been updating , improving and modifying m&p pistols and other brands for years .

 

[Zebra War Wagon]

I have a FS 2.0 in 40 and the Storm Lake 357 Sig barrel as well as a Compact 2.0 in 40 with a Storm Lake 357 Sig barrel and a 9mm conversion barrel. I had my local smith cut the barrels down to 4" so they fit flush with the slide, and they also have an 11 degree crown.
I have fired both extensively with all the capable calibers, and not a problem or any excessive wear.
The only difference I notice is that when firing the 9mm with the 40/357 Sig recoil spring, the slide movement is a lot slower, but 100% functional.
I will continue to shoot and carry either of these pistols without hesitation or concern.

 

[thewittmp]

"This is not something we recommend as there is no ability to make conversions among our firearms. You cannot swap barrels and slides between the M&P9, M&P40 or .357 Sig. Doing so is very dangerous, as the slides and barrels are not the same. None of the M&P pistol line is designed to change calibers by changing barrels and magazines. Each pistol is designed to only shoot the cartridge stamped on the slide and barrel as shipped by Smith & Wesson."

From the text, even though they state "conversions" at the beginning, the rest of the text seems to reference simply swapping calibers.

Conversion barrels have the correct dimensions for their host caliber, so they do match the slide.

 

[Walt Sherrill]

As has been noted in earlier messages, the ONLY difference between the .40 version of the M&P and the .357 SIG version is the barrel. Period.

A 9mm gun's slide would be substantially different than a .40/.357 SIG slide, because the diameter of the .40/.357 Sig barrel is greater. You can't put a .40 barrel into an M&P 9 slide.

Some gunmakers use the same slide for a number of different models and have different recoil spring assemblies to compensate for the the different slide velocities; others modify the slide to make it heavier or lighter and use the same recoil assembly for all models that are the same general size. (Glock does this.) With SIG .40/.357 SIG models, the only differences in the slides are the sights.

I've never found a .40 > 9 "conversion" barrel that would work in a 9mm slide. As far as I know nobody making aftermarket conversion barrels even offers that type of "smaller-to-larger caliber" type of conversion option.

For guns like the small-framed Witness (Tanfoglio) line, the barrels for the 9mm models are the same diameter as the .40/.357 SIG barrels. The barrels were interchangeable -- as were the 9mm and .40 magazines, but matching the caliber of the magazine to the barrel being used ran better, as 9 and .40 mags were not the same.

My .40 rounds, when used in a 9mm mag, would "clunk" as they fed, hitting the feed ramp differently and probably bumping the front edge of the magazine as they fed. (The gap at the top of the 9mm mag is narrower and the round sets lower in a 9mm mag).

Now that Witness no longer imports the smaller-framed models to the U.S. and only large-frame models are available, the slides will interchange between frames and calibers, but not all barrels will. All of the large-frame models -- including the .22 -- use the same "form factor" for their magazines, but with spacers fitted (and not removable) that vary depending on the caliber being used.

 

[350mag]

S&W CS says M&P 40 to 357 sig conversion unsafe.

I am fortunate enough to have an original 357 Sig M & P. I bought extra M & P guns in 40 and 9, instead of buying extra barrels. I enjoy each one. The main difference between the 40 and the 357 Sig is the barrel and then the caliber designation on the side.
Sent from my iPhone using Tapatalk

 

[michaelstreet246]

.357 pressures

I'm pretty sure a lawyer wrote that response. I have converted my .40 to 9mm with no issues, and since the .40 and .357 SIG share the same magazines and rear case dimensions, you shouldn't have any difficulty at all.

Do a search for M&P conversions and I'm sure you'll have lots of material to study.

Welcome to the forum, by the way.


ETA: Pressures in the .357 SIG are indeed substantially higher than in the .40, but based on the fact that S&W produced the M&P in .357 SIG,
I'd say it's doable, but may accelerate wear on the firearm.

I ordered the .357 barrel for my M&P .40 I heard all of the noise and had to see for myself. I used a micrometer and the dems. were the same except for the ID, someone once said that they would work but that the 40 would hit low, I said how can that be when they are both centerfire. Anyway I dropped the barrel in and it fit perfect and shot nicely. I hear the talk about pressure differences but if you are using the barrel meant for the round then how is that going to make a differe
nce?