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Smith & Wesson M&P Pistols All Variants of the Smith & Wesson M&P Auto Series


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Old 05-08-2017, 03:48 PM
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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Default S&W CS says M&P 40 to 357 sig conversion unsafe.

I've used 1911's for decades but I'm new to the M&P...and this forum. I recently purchased an M&P 40 (4.25" barrel, original version not M2.0) and I've been looking into buying the S&W 357 sig barrel available at Midway to convert the pistol to 357 sig. I've read many positive comments from others who made the simple drop in conversion but to be thorough I emailed S&W Customer Service to ask about the possible need for a heavier recoil spring or likelihood of accelerated pistol wear and I got this discouraging response.

"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."

Is S&W being overly cautious due to liability concerns or is this drop in barrel conversion in fact "very dangerous"?

Last edited by Cruiser55n; 05-08-2017 at 04:01 PM.
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Old 05-08-2017, 03:54 PM
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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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,
Iíd say itís doable, but may accelerate wear on the firearm.

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Old 05-08-2017, 04:44 PM
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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Certain dimensions may be similar, or even exactly the same.

However, PRESSURE, which is a major factor in function, wear and safety, is different enough that S&W no longer offers a specific .357 SIG product in the M&P line and recommends not adapting a lower pressure-designed gun to run a higher pressure cartridge--or do it at your own risk.

IMO, the risks and liabilities associated with such a caliber change just aren't worth it. Fans of the .357 Sig caliber will disagree, believing it has certain positive characteristics. I have been there and done that with another manufacturer's pistol with their own manufactured barrel, and it busted up the weapon. I gained so little in performance and so much in cost and aggravation that I won't do it again.
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Old 05-08-2017, 04:59 PM
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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How are they gonna sell you another gun if you convert the one you have? That and insulating themselves from liability. I don't know of many manufacturers of any kind that will approve modifying their product.

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Old 05-08-2017, 05:18 PM
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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I don't believe it's a CYA situation on Smith's part. If you were taking a 357 Sig gun and converting it to 40 that would be one thing. The 357 Sig is a significantly higher pressure round than 40 S&W. Not a good idea in my opinion, for what it's worth.
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Old 05-08-2017, 05:21 PM
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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Quote:
Originally Posted by CB3 View Post
Certain dimensions may be similar, or even exactly the same.

However, PRESSURE, which is a major factor in function, wear and safety, is different enough that S&W no longer offers a specific .357 SIG product in the M&P line and recommends not adapting a lower pressure-designed gun to run a higher pressure cartridge--or do it at your own risk.

IMO, the risks and liabilities associated with such a caliber change just aren't worth it. Fans of the .357 Sig caliber will disagree, believing it has certain positive characteristics. I have been there and done that with another manufacturer's pistol with their own manufactured barrel, and it busted up the weapon. I gained so little in performance and so much in cost and aggravation that I won't do it again.
Actually, I'm a big fan of 357 SIG but I agree with you totally. The pressure, slide speed and so on just exceed what the firearm was designed to withstand.

I doubt the firearm would detonate or fail catastrophically, but it'd probably do some damage in the long run. And if a person is willing to take that risk and pay that cost, who am I to argue with them?
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Old 05-09-2017, 01:41 PM
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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If I recall correctly, S&W once sold a .357 barrel along with the MP40 as a kit.
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Old 05-09-2017, 02:00 PM
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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Quote:
Originally Posted by Cruiser55n View Post
I've been looking into buying the S&W 357 sig barrel available at Midway to convert the pistol to 357 sig. I've read many positive comments from others who made the simple drop in conversion
I did exactly this and have run about 300 rounds of 357 SIG, both FMJ and HP without any issues. I found that the 357 SIG round was too hot for me in a polymer gun. I am fine shooting the round in my all metal SIGs, but it's just not for me in polymer. I also bought a Storm Lake 9mm conversion barrel for my M&P40 and it runs like a champ using 9mm mags. Hope this helps.
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Old 05-09-2017, 02:21 PM
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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I had a Gen 2 Glock 23 .40 that I wanted to convert to .357sig. Was told that the Gen 2 frame would not support it. Upgraded to a Gen 3 and had no issues. I'm sure S&W is being cautious for the same reason. Perhaps they know the frame is not up to the task. The barrels themselves are designed to drop in. I think it's the pressure exerted on the frame being on the upper limits of safe.
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Old 05-10-2017, 04:25 PM
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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Sounds like they're simply giving the answer provided by they're legal team. The only differences on those pistols is the barrel itself. The frame, springs, and slide are the same. It's a mass produced pistol made to hit a fairly low price point so they're not investing in making separate frames for the 357s. If they did we'd see a difference in price.

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Old 05-10-2017, 04:45 PM
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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I have an M&P .357SIG that also has a 9mm and a .40S&W barrel. The fact that it was originally designed and manufactured for the .357SIG makes it capable of handling the other two calibers without any problem. I am not sure that the 9mm and .40S&W couldn't also handle the .357SIG.
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Old 05-11-2017, 02:22 AM
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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Thanks to everyone for sharing your experience. Subsequent to my original post I also received a response from Storm Lake advising against firing 357 SIG with the M&P 40.

Although I get the sense the danger is actually minimal and limited use of 357 SIG wouldn't do substantial harm to the weapon I'm going to err on the side of caution given that safety is paramount when employing firearms. 357 SIG sounds like a terrific cartridge but 40 S&W will do for my purposes.
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Old 05-11-2017, 12:14 PM
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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I think you are wise, Sir! S&W didn't just arbitrarily make that judgement about the use of their product. There had to be some events that they are aware of that resulted in this statement. Yes, it probably has to do with liability as well. Folks do a lot of things that they want to do in spite of common sense reasons not to do so. Many of them seem to get by with these things and so they highly recommend doing so. You pay your money and you take your chances, but if you go beyond the advice of the manufacture, then the responsibility for any negative consequences is on you. Sadly, the manufacturer gets the blame in most cases.

I've seen reloaders exceed all the printed and recommended bounds for hot loading cartridges with sooner or later negative consequences. Even if there is no kaboom, the accelerated wear and tear on the weapon platform is unavoidable. And if I happen to buy one of these stretched and overstressed guns on the used gun market, well ... I'm not happy!

Each of us must make our own choices and take our own chances. Yes, I have fired my own reloaded ammo in several guns over the years. But I am not interested in doing anything but maybe creating a load that is within the recommended bounds of safety that is maybe also a bit more accurate or less expensive than the ammo available from factory producers with a good reputation. I do not allow my reloads to be shot by anyone else, and I DO NOT shoot anyone else's reloads knowingly. I also do not quietly sell any gun that I know might be suspect in fit or function. I like to sleep at night. I am very careful and cautious about taking anyone's advice about this sort of thing, especially when I see that they have done something without any kind of real testing beyond "my gun hasn't blown up yet!"

I'm just stating my own policies here, having gotten some experience the hard way and having tried hard to learn from the negative experiences of others along the way. Manufactures who sell products to make money do not advise against something that they could make money doing for no good reason. Sadly, there are a few who are only interested in making money and will not do this because they know some folks will buy their product thinking that if it's for sale, it's gotta be safe and good. Me personally, I just like to err (if I do that) on the side of caution! But like John Wayne famously stated in one of his movies, "I think a man oughta do what he wants to do!", even when he has been cautioned against it. Just give me time to get out of the way!!!
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Old 05-13-2017, 01:31 PM
Walt Sherrill Walt Sherrill is offline
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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I don't know if anyone has paid attention to it, but S&W discontinued production of their M&P-based .357 models some time ago. CB3 mentions this, in an earlier (#3) response above.

I previously wrote on this forum about how the .357 SIG M&P's used by the North Carolina Highway Patrol were having problems, and that S&W was unable to resolve them. The NCHP eventually gave their M&Ps to S&W and switched to SIG P229s.

The information I posted on this forum about that situation was based on first-hand info from a son who is a NCHP Trooper, from new articles from the Raleigh (the state capital) newspapers at the time, and press releases from the NCHP; it wasn't too much later that S&W discontinued production of all M&P .357 models. If you check the website, you'll see that they are no lnger available.

It may be "lawyer talk but it may be for a good reason.

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Old 05-14-2017, 11:36 AM
Bleek Gilliam Bleek Gilliam is offline
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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Wasn't the issue they were having dead triggers related to the 1st generation sear block assemblies? They discontinued them rather than fix the problem with the updated sear housing blocks.

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Old 05-14-2017, 12:38 PM
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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Perhaps the new 2.0 frame is more robust, and it has updated internals. Maybe this new model could handle .357 SIG better?

However, until S&W brings out their own new .357 SIG and while they continue a policy of saying do not convert to this caliber, all safety issues, wear & tear and warranty issues are the responsibility of the shooter.

As always: Follow the money. This never was a very high return on investment caliber for manufacturers. It continues to fade in popularity with LE and never was an adopted military caliber. S&W is doing fine financially without it, hardly pissing anyone off by not messing with it.

My prediction: you won't see major manufacturers offer a pistol chambered specifically for it again--or condone a conversion to it--other than SIG, which will do it for reputation rather than financial purposes.

The supposed feeding advantage to a bottle neck cartridge is a non-issue, as the other major cartridges feed just fine.

Flatter shooting for self defense? Non-issue out to 50 yards.

Higher velocity with deeper penetration? Not needed compared to other catrtridges with modern bullets meeting SD requirements.

Expensive. Hard to find. Limited loading choices. More recoil and blast, making it harder to shoot well and fast. Eats up gun and shooter. Lower capacity vs 9 mm.

What's to like?

Last edited by CB3; 05-14-2017 at 12:40 PM.
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Old 05-14-2017, 01:53 PM
Walt Sherrill Walt Sherrill is offline
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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Quote:
Originally Posted by CB3
Flatter shooting for self defense? Non-issue out to 50 yards.

Higher velocity with deeper penetration? Not needed compared to other catrtridges with modern bullets meeting SD requirements.
Likely true on both counts. Some LE Agencies, however, do prefer the .357 round to other rounds because they feel that round is more effective when penetrating windshields on moving cars. (That's the theme I've heard a number of time. Whether its valid or not remains open.)

Quote:
Originally Posted by Bleek Gilliam
Wasn't the issue they were having dead triggers related to the 1st generation sear block assemblies? They discontinued them rather than fix the problem with the updated sear housing blocks.
Yours is the first description I've seen of the problem being identified. Seems as though fixing the problem should not have been that difficult or expensive The last I heard -- which was several years ago -- a solution had not been found.

A replacement Sear Block Assembly is about $25, retail, S&W's cost would be a lot less. Unless a major gun redesign was needed to resolve the problem, upgrading all .357 SIG models out in the field could have been expensive, but not something that would have driven them into a bad years -- given how well some of their guns have been selling.

As CB3 said earlier, it makes sense to follow the money...

I wonder if that assembly in the M2 models is much different or how that portion of the frame is seemingly upgraded? If it is, we might see new .357 SIG S&Ws in the future.

Last edited by Walt Sherrill; 05-14-2017 at 03:16 PM.
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Old 05-14-2017, 03:51 PM
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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I started with a .357 SIG, and not long ago bought the 9mm barrel and one magazine from Midway. Lets me shoot the gun for a lot less cost. But I would have hesitated to go the other direction.
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Old 05-14-2017, 08:09 PM
Walt Sherrill Walt Sherrill is offline
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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Quote:
Originally Posted by hyena
I started with a .357 SIG, and not long ago bought the 9mm barrel and one magazine from Midway. Lets me shoot the gun for a lot less cost. But I would have hesitated to go the other direction.
In most cases, if you started with a 9mm gun, you can't go in the other direction. There aren't any conversion barrels that allow it; it's almost always larger caliber to smaller only. (There may be exception, but they're rare.) Most guns that can handle the .357 SIG round are a bit more robust and often have heavier slides than similar models shooting the 9mm round.

I have an FNS-40 and picked up a 40> 9mm conversion barrel for it. I have also done that with a number of others makes over the years. Sometimes the .40 mags will work with 9mm rounds and sometimes they won't. With my FNS-40, 9mm hardball worked fine in the .40 magazine, but open "mouth" of at least one brand of hollow points would hang up on or bite the feed ramp. They seem to start at a lower point on the feed ramp when being fed from a .40 mag with the FNS line.)

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Old 05-14-2017, 08:37 PM
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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I must be a simpleton. If I wanted to shoot SIG 357 well then I would buy a SIG 357 firearm. I am not comfortable with the conversion kits. I feel a lot more confident shooting factory condition firearms. Good luck!
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Old 05-14-2017, 11:35 PM
Walt Sherrill Walt Sherrill is offline
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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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.

Last edited by Walt Sherrill; 05-15-2017 at 10:04 PM.
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Old 05-15-2017, 01:13 AM
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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Guess I'm just living on the edge
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Old 05-26-2017, 11:28 AM
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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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.
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Old 05-27-2017, 11:22 PM
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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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.
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Old 05-31-2017, 12:22 PM
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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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.
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Old 05-31-2017, 02:10 PM
Walt Sherrill Walt Sherrill is offline
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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Quote:
Originally Posted by gojones
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.
.

Last edited by Walt Sherrill; 05-31-2017 at 02:11 PM.
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Old 05-31-2017, 02:30 PM
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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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.
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Old 05-31-2017, 02:36 PM
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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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
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Old 05-31-2017, 10:47 PM
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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Still waiting on SG to get that Speer Lawman back in stock for $16 a box. Thats hard to beat.
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Old 03-29-2020, 04:51 AM
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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Default 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
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Old 03-31-2020, 10:00 PM
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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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
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Old 03-31-2020, 10:32 PM
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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Quote:
Originally Posted by Blww View Post
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
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Old 04-01-2020, 08:55 AM
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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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 .
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Old 04-01-2020, 09:29 AM
HOUSTON RICK HOUSTON RICK is offline
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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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.
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Old 04-02-2020, 07:33 AM
hardluk1 hardluk1 is offline
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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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 .
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Old 04-02-2020, 09:48 AM
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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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.
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Old 04-02-2020, 11:29 AM
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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Quote:
Originally Posted by Cruiser55n View Post
"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.
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Old 06-30-2020, 01:22 PM
Walt Sherrill Walt Sherrill is offline
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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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.

Last edited by Walt Sherrill; 06-30-2020 at 01:34 PM.
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Old 07-28-2020, 11:30 PM
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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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.


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Old 10-04-2020, 07:51 PM
michaelstreet246 michaelstreet246 is offline
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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Cool .357 pressures

Quote:
Originally Posted by scattershot View Post
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,
Iíd say itís 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?
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Old 10-07-2020, 03:48 PM
Oldsalt66 Oldsalt66 is offline
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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Same frame, same firing pin, same recoil spring assembly, same striker, same magazine.....same internal parts, although I canít swear for the locking blocks being identical.

Has anyone heard of any real world issues regarding this matter ?

I, myself, have not and Iíve been messing around with polymer M&Ps since their introduction, which btw, was in .40 S&W making them robust pistols.

ETA:

The following pertains only to legacy models and not the 2.0 line with which I have no experience.

You can replace the barrel of an original model .40 S&W M&P with a factory 9mm barrel and the result will be a perfectly shootable pistol.

However, the opposite is not true; you cannot replace the barrel of a 9mm M&P with a factory .40 S&W barrel.

The conversion between .40 and 9mm goes only one way; .40 to 9mm.

Last edited by Oldsalt66; 10-07-2020 at 04:05 PM.
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Old 10-08-2020, 06:22 PM
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S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe. S&W CS says M&P 40 to 357 sig conversion unsafe.  
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I have a LE trade in M&P .40 with a .357 SIG barrel in it. I have been shooting it now for over 3 years without ANY problems. I must be living on the edge.
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