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Discussion Starter #1 (Edited)
Happy Friday my fellow M&P friends! So I've done my due diligence in searching the net and reading about what one needs to do in order to make this happen, but I just wanted to make a quick post here to confirm so that nothing gets done incorrectly. And since a lot of what I read was from forum threads posted a while back, I wanted to make sure that nothing has really changed and I'm still current. Because you know, times change, and the answer given in one thread may no longer be the "right" answer. Technology, market forces, the regulatory environment, manufacturing, culture, etc. can all cause changes, either for better or worse. You get what I'm sayin'.

Many people like to convert a M&P40 to 9mm so that they have 2 guns in one. From what I've gathered, you basically only need to do 2 things:

1. Drop in a 9mm conversion barrel
2. Use 9mm magazines (40 mags will work just fine for the most part but some people say that 9mm mags tend to work more consistently).

However, when it comes to the barrel, from what I've gathered, there are "regular" 9mm barrels, and then there are "conversion" 9mm barrels. The regular barrels are for 9mm guns/slides, and are more or less just replacement barrels for an existing 9mm handgun. Whereas the conversion barrels are made specifically for someone trying to convert from a larger caliber (40S&W or .357Sig) down to a smaller caliber (e.g. 9mm). And correct me if I'm wrong, but this is because the M&P9 and M&P40 have different internal measurements and tolerances and thus the M&P40 slide probably shouldn't use a 9mm barrel that is made for a 9mm slide (although some people do it anyway). Therefore there are aftermarket 9mm conversion barrels made specifically to be dropped into a 40S&W slide, and these conversion barrels are an exact (sort of) fit, whereas the regular 9mm barrels are not. This is because in the M&P40 the ejector, extractor, breach face, and barrel crown, etc. are all different than the M&P9, and the conversion barrels take these differences into account. Yes/no/sort of???

Also on the 9mm conversion barrels, you have the choice between a pre-fit drop in barrel, or a barrel that will need to be fit by a gunsmith. And from what I've read even further, there's probably no need to get it fit by a gunsmith unless I am a tournament shooter and want 2" groups at 25 yards. Which I'm not, and realize that is unrealistic for me and a pre-fit drop in barrel will probably be more accurate than I am.

So that's what I've read and assume to be correct. Please correct me if I'm wrong on any of this or if I am missing something important. And if you have anything of value to add that I did not mention, please do tell!
 

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I believe you have the correct info sir.

All I can add is that the most popular conversion barrels are Storm Lake and KKM. I know that the Storm Lake is a drop in part, and I believe the KKM is as well.
 

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i have a 40FS and wanted to do this.
i did a lot of reading and research. i read many who have felt comfortable with just dropping a OEM 9mm barrel in their 40.
i decided i didn't want to risk it for the difference of $50-$70 and went with a SL 40 to 9 conversion barrel.
i am very happy and pleased with it and feel i made the right choice for my own piece of mind.
i have put about 1000 rounds of 9mm through it without any issues at all.
i was lucky to find a local gun store that just happened to have one sitting around, when there were none to be found, last year. the OEM 9mm barrels were not to be found either. the SL one luckily showed up first and i jumped on it.

highly recommend the SL conversion barrel.
 

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Discussion Starter #4
Okay, good to know. My next question was what are my choices for 9mm conversion barrels. Are there any others besides Storm Lake and KKM?
 

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APEX has been working with Bar-Sto, you might dive those 2 a look.
 

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APEX has been working with Bar-Sto
They first mentioned this collaboration back in 2011 and it has been a while since anything else has been said about it. AFAIK the project is dead.
 

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I have a Storm Lake in mine and it shoots bug holes. A "custom fit" would have been an absolute waste of money....

Other than a couple of bonks in head from ejection, it's one of the best/softest 9MM I've ever shot.

Mine was bought in .357 sig and what that round does to coyotes is amazing!
 

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I've become somewhat of an M&P addict. I run an 9FS for IDPA, 9L for Production USPSA and a 40L (Customized) for Limited USPSA. My carry is a M&P Shield 9.

Wanted to convert the 40L to 9mm for the occasional steel matches, since the 40 has a brass magwell, tungsten rod, racker, nice FO sights, and race holster setup.

The lowers of ALL M&P 9/357/40's are identical. There is no difference in frame mechanism design or dimensions, other than the generational changes over the years and various regional differences, such as thumb or magazine safeties.

However, the slide breach dimensions, barrel, magazines and recoil spring are the ONLY differences between the calibers. The key point about conversion barrels is that the breach dimension of the chamber hood matches the 40 slide.

I went with the KKM because they are a local business to me, and they ship direct within a day. Also, they use the same chamber design as used on the late models S&W barrels.

Dropping in a stock 9mm barrel works, but there is extra play between the barrel/slide breach interface which can make for some loss of accuracy at long distances, like 25 yrds or more.

Some have said that S&W doesn't warrant any problems might be caused by NOT using a conversion barrel. Not sure if this is true.



 

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Happy Friday my fellow M&P friends! So I've done my due diligence in searching the net and reading about what one needs to do in order to make this happen, but I just wanted to make a quick post here to confirm so that nothing gets done incorrectly. And since a lot of what I read was from forum threads posted a while back, I wanted to make sure that nothing has really changed and I'm still current. Because you know, times change, and the answer given in one thread may no longer be the "right" answer. Technology, market forces, the regulatory environment, manufacturing, culture, etc. can all cause changes, either for better or worse. You get what I'm sayin'.

Many people like to convert a M&P40 to 9mm so that they have 2 guns in one. From what I've gathered, you basically only need to do 2 things:

1. Drop in a 9mm conversion barrel
2. Use 9mm magazines (40 mags will work just fine for the most part but some people say that 9mm mags tend to work more consistently).

However, when it comes to the barrel, from what I've gathered, there are "regular" 9mm barrels, and then there are "conversion" 9mm barrels. The regular barrels are for 9mm guns/slides, and are more or less just replacement barrels for an existing 9mm handgun. Whereas the conversion barrels are made specifically for someone trying to convert from a larger caliber (40S&W or .357Sig) down to a smaller caliber (e.g. 9mm). And correct me if I'm wrong, but this is because the M&P9 and M&P40 have different internal measurements and tolerances and thus the M&P40 slide probably shouldn't use a 9mm barrel that is made for a 9mm slide (although some people do it anyway). Therefore there are aftermarket 9mm conversion barrels made specifically to be dropped into a 40S&W slide, and these conversion barrels are an exact (sort of) fit, whereas the regular 9mm barrels are not. ****This is because in the M&P40 the ejector, extractor, breach face, and barrel crown, etc. are all different than the M&P9, and the conversion barrels take these differences into account. Yes/no/sort of???****

Also on the 9mm conversion barrels, you have the choice between a pre-fit drop in barrel, or a barrel that will need to be fit by a gunsmith. And from what I've read even further, there's probably no need to get it fit by a gunsmith unless I am a tournament shooter and want 2" groups at 25 yards. Which I'm not, and realize that is unrealistic for me and a pre-fit drop in barrel will probably be more accurate than I am.

So that's what I've read and assume to be correct. Please correct me if I'm wrong on any of this or if I am missing something important. And if you have anything of value to add that I did not mention, please do tell!
ALMOST perfect...

The breach face and ejector/extractor, etc are all the same on the 40 and the 9.

These differences exist:
1. In the rear portion of the slide, the notch at the back of the chamber hood is wider on the 40 slide, hence the external differences in the barrels (pictured above). A 40 barrel will NOT go into battery in a 9 slide, and a 9 barrel will not lock up completely in a 40 slide.
2. The feed lips are wider on a 40 magazine. If you shoot 9mm in a 40 mag, the rounds will sit too high in the magazine and should cause feeding issues. 40 rounds won't feed through 9 mags since the lips are too narrow (for the narrower 9 case.)

To shoot 9mm SAFELY in a 40, you will need a conversion barrel (external dimensions of a 40 barrel, but with 9mm chamber and barrel), and 9mm magazines.

Otherwise the 9mm and 40 are identical, and share the same part numbers, including recoil springs, extractors, ejectors, sears, everything.

JeffWard
 

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ALMOST perfect...

The breach face and ejector/extractor, etc are all the same on the 40 and the 9.

These differences exist:
1. In the rear portion of the slide, the notch at the back of the chamber hood is wider on the 40 slide, hence the external differences in the barrels (pictured above). A 40 barrel will NOT go into battery in a 9 slide, and a 9 barrel will not lock up completely in a 40 slide.
2. The feed lips are wider on a 40 magazine. If you shoot 9mm in a 40 mag, the rounds will sit too high in the magazine and should cause feeding issues. 40 rounds won't feed through 9 mags since the lips are too narrow (for the narrower 9 case.)

To shoot 9mm SAFELY in a 40, you will need a conversion barrel (external dimensions of a 40 barrel, but with 9mm chamber and barrel), and 9mm magazines.

Otherwise the 9mm and 40 are identical, and share the same part numbers, including recoil springs, extractors, ejectors, sears, everything.

JeffWard

+1
 

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ALMOST perfect...

The breach face and ejector/extractor, etc are all the same on the 40 and the 9.

These differences exist:
1. In the rear portion of the slide, the notch at the back of the chamber hood is wider on the 40 slide, hence the external differences in the barrels (pictured above). A 40 barrel will NOT go into battery in a 9 slide, and a 9 barrel will not lock up completely in a 40 slide.
2. The feed lips are wider on a 40 magazine. If you shoot 9mm in a 40 mag, the rounds will sit too high in the magazine and should cause feeding issues. 40 rounds won't feed through 9 mags since the lips are too narrow (for the narrower 9 case.)

To shoot 9mm SAFELY in a 40, you will need a conversion barrel (external dimensions of a 40 barrel, but with 9mm chamber and barrel), and 9mm magazines.

Otherwise the 9mm and 40 are identical, and share the same part numbers, including recoil springs, extractors, ejectors, sears, everything.

JeffWard
+2! these are the points that made the decision for a true conversion barrel for me, instead of an OEM 9mm barrell.
 

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Great response Jeff.... you took the words right off of my keyboard!!
 

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Discussion Starter #13
ALMOST perfect...

The breach face and ejector/extractor, etc are all the same on the 40 and the 9.

These differences exist:
1. In the rear portion of the slide, the notch at the back of the chamber hood is wider on the 40 slide, hence the external differences in the barrels (pictured above). A 40 barrel will NOT go into battery in a 9 slide, and a 9 barrel will not lock up completely in a 40 slide.
2. The feed lips are wider on a 40 magazine. If you shoot 9mm in a 40 mag, the rounds will sit too high in the magazine and should cause feeding issues. 40 rounds won't feed through 9 mags since the lips are too narrow (for the narrower 9 case.)

To shoot 9mm SAFELY in a 40, you will need a conversion barrel (external dimensions of a 40 barrel, but with 9mm chamber and barrel), and 9mm magazines.

Otherwise the 9mm and 40 are identical, and share the same part numbers, including recoil springs, extractors, ejectors, sears, everything.

JeffWard
This is basically what I said, except for my mistake of thinking the breach face and ejector/extractor were the same. But as far as the reason as to why to get a conversion barrel over a regular barrel, I covered in my OP.

ALMOST perfect!

I feel like we should sticky this so people aren't throwing stock 9 barrels in their 40 slides.
 

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This is basically what I said, except for my mistake of thinking the breach face and ejector/extractor were the same. But as far as the reason as to why to get a conversion barrel over a regular barrel, I covered in my OP.

ALMOST perfect!

I feel like we should sticky this so people aren't throwing stock 9 barrels in their 40 slides.
Some will do it anyway. They had a friend, or so and so says, or it's worked so far, etc., etc. :no
 

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Discussion Starter #15
Oh I know they most certainly will. As the saying goes, "There's one in every crowd." But is it really worth saving yourself ~$55 by throwing in a stock OEM barrel, when the fit tolerance won't be as tight, and therefore you greatly risk suffering from subpar accuracy and even kabooms!?

I always say if you're gonna do it, do it right.
 

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I have a wonderful solution: By one of each and enjoy the dilemma! I own a 9mm and 2 40 cal and I love them all like children!
 

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The Storm Lake conversion barrel fits perfectly. I put a caliper on the two barrels and they are identical in dimensions. There is a slight difference in the feed ramp. Appears that the SL conversion barrel is slightly wider. Also a slight difference at the base of the forward lock-up tang.

Here's some comparison pictures between the stock 40 cal and SL 9mm conversion barrels.
 

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Just saying thanks...

Just saying thanks to everyone who contributed to this discussion.. Bought my first M&P recently and was thinking of converting from 40 to 9 for practice and to help reduce target shooting costs. The information in this thread answered a lot of questions and concerns I had, and helped me make the decision to "pull the trigger" on getting a conversion barrel.
 

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COME ON STORM LAKE, MAKE A 9MM CONVERSION BARREL FOR THE SHIELD!!!

IT CAN'T BE THAT HARD!!
 
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