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"?

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.

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.

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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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.

CB3 said:

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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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?

If I recall correctly, S&W once sold a .357 barrel along with the MP40 as a kit.

Cruiser55n said:

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

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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.

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.

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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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.

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.

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!!!

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.

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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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?

CB3 said:

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.

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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.)

Bleek Gilliam said:

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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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.

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.

hyena said:

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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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.)

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!