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ig MPX owners: Please help!

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After throwing an embarrassing amount of time and money at the problem of jams with my MPX, I finally determined that I have a problem with extraction. Specifically, my gun seems to wear out/destroy extractor springs extremely fast. After only a few magazines, the spring is much shorter, and extractor tension is greatly reduced. In this picture, on the left is a brand new MPX extractor spring and insert that I ordered directly from Sig. On the right is the same spring after 100 rounds. I understand a spring will take a set, but it lost somewhere between 1/4 to 1/3 of its length. 

1. Is that normal? 

2. If it's not normal could somebody check a few things on your bolt and extractor? The depth of the hole in the bolt body that the spring fits down into is .168 deep measured from the top of the bolt right behind that machined out area. The depth of the hole that the spring fits into in the extractor itself is .0685. I'm wondering if that might be out of spec and the spring doesn't have as much room as it should and is being crushed? 

I have tried various springs including standard Sig MPX springs, and various AR springs (standard and extra power). All of them have good tension to start with, but then after a very few rounds the spring is flattened to the point the that the extractor is just "flopping around" almost and doesn't have the strength to pull the case out causing all kinds of problems. 

I have not tried using a donut o-ring yet, so I may try that, but IMO you should not NEED something like that just for a gun to run normally. 

BTW sending the gun to Sig is basically not an option. It has an aftermarket barrel and gas block (which I have a feeling they would balk at) and it's an SBR and I don't want to deal with the risk of shipping the thing. The aftermarket barrel and gas block have no relation to the extractor problem, as the gun did the same thing with the factory parts too.

extractor spring sm.jpg

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I don't have the factory parts any more. For numerous reasons in addition to the the ones I've given, sending it back to Sig is not really an option.


If somebody has an MPX and could check those measurements, at least then I'll know whether my parts are out of spec and I can go from there. Thanks!

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Perhaps look into replacing the extractor itself?  Shooting Sports Innovations and ILWT are carrying a billet machined extractor as an improved factory replacement.  My factory spring went about 6000 rounds before it broke in half and stopped ejecting (in the middle of a match, of course.)  A Taccom replacement just failed last week after about 3000.  Another club shooter I've known for years has a 2017 Gen 1 that he's NEVER replaced any springs on, and it's got many tens of thousands of rounds though it.

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I am definitely getting the spring all the way into the extractor. Seems like the measurements on my bolt/extractor are close to others'. It does look like my spring has taken a bit more of a set though especially after only 100 rounds, and I know from past experience with this gun that if I keep shooting it will only get squished further.


This is a gen II and I have shot a mix of handloads and cheap range ammo (Winchester and Blazer Brass). The extraction problems are present for all types of ammo.


I've already spent a lot of time and $$$ trying to get this thing to run, so what's another $50? I might as well try that SSI/ILWT extractor. At this point this gun is about 50% ILWT anyway - barrel, gas block and adjustable gas plug.

Does anybody use the little donut o-ring in their MPX in addition to the spring/insert? 

Edited by hhsmiley
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6" barrel


Muzzle device is a pretty basic 2 side port brake similar to this pic but without the ports on top.


Ammo varies but my handloads are about 1000 fps with 124 gr and the range ammo is 115 gr at around 1200 fps.


I'm not sure which setting the plug is on. There are no external markings or way that I can see to determine that. When I installed it, I just rotated it around to different settings until I got the gun to function, and placed a witness mark on the plug and gas block so I'll know where to set it if I ever change it for some reason.


When it does extract, my brass is landing in an area about 5-6 ft away slightly forward at about my 2 o'clock. Does that sound normal? 


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Check the ejector in the upper, make sure it is tightened down... on the gas plug that's why i went with the fixed port plugs from ILWT so I would know what I am using in each gun... What recoil springs  ( stock, XP, reduced)? I have been happy with the SSI / ILWT billet extractor in all of my MPX's.

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I checked the ejector. It's in there good. No movement or anything like that.


I'm using the original stock Sig recoil springs. I only have maybe 600-700 rounds through this gun (bought in 2016), so I didn't think they would be anywhere close to needing replacement. I just haven't shot it much because it's been a problem almost from the get go.


Most of the time I can pretty well work out a problem and solution to a gun issue but this one has me stumped. I just know I've never been able to get through more than 3-4 mags without the extractor losing tension.


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My 8" SBR also does the same thing. I'll get maybe 200-300 rounds and inspect the spring, and it'll be squished like yours.

When I replace it with a new spring, my gun will start out the first 6-12 rounds with weak ejection, then progressively building to a fair pattern - about 8 feet away at 2-3 o'clock.


8" ILWT barrel

.0520" ILWT gas plug

ILWT piston

ILWT/SSI extractor

ILWT firing pin and spring

I don't roll my own, so I run Federal Syntech 150gr. Softest factory ammunition that runs consistently and reliably in my MPX.


Sig and ILWT extractor springs only last about 200-300 rounds before they look squished and I replace them before every match.

However, I just started to run BCM extra power springs and they seem to last about double that so far. 



Edited by SergioSF
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I'm at 15K rounds and all my internals except the trigger are stock and I have had no issues at all. My extractor spring still looks new. I'm wondering if there is a correlation between the guns that have had extractor problems and those that have opened up their gas ports. I guess it could be possible that they are being stressed by trying to extract too soon while the pressure is still up, just spit balling here since some of the factory guns have no trouble at all like mine and it's not been messed with.


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Try the BCM extra power kit and also install the o-ring. See how it runs. I have one of my later gen2 bolts that refuse to extract reliably without it. I have another earlier gen2 bolt that is fine with ILWT spring and insert only. I have a third one that’s the latest PCC model and they came with a red heavier spring and o-ring installed. I left it as-is for now.

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1 hour ago, jlo86cj said:

Try the BCM extra power kit and also install the o-ring. See how it runs. I have one of my later gen2 bolts that refuse to extract reliably without it. I have another earlier gen2 bolt that is fine with ILWT spring and insert only. I have a third one that’s the latest PCC model and they came with a red heavier spring and o-ring installed. I left it as-is for now.


I've tried this (with the o-ring)... but couldn't get the pin back in due to so much tension. If you were able to do this, I'll give it another try. 

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At one point I sent my upper to Rob @ ILWT to see what the problem was and he was not really able to replicate the problem. He very graciously did that for free and didn't charge me anything for looking at it or for ammo for testing. It's possible they just didn't put enough rounds through it to get the spring to the point of getting smashed and causing problems. At this point I am hesitant to bug him further with this because he's already gone way above and beyond IMO. ILWT is very GTG, both their parts and service.


I have tried a Wolff extra power AR extractor spring, standard AR springs, MPX springs from ILWT, and factory Sig MPX springs. All have been smashed. The Wolff had too many coils and I could not get the extractor pin back in with that. I did try cutting off one coil and that spring just got smashed like all the others. Curiously, the Wolff springs would be completely compressed and not allow the pin to go into my AR bolts too, so not sure what's up with that.


At this point, I'm going to order the ILWT extractor. I'm afraid I also won't be able to get the extractor pin back in with the BCM kit but I'll go ahead and try that too.


Based on the responses, I don't think it's an out of spec bolt or extractor, but maybe there's something to the over gassed thing. The only problem with that is that I have the adjustable gas plug set on the lowest gas setting that will cycle the gun. Any less gas and it doesn't work reliably. 


I just got done building a 9mm AR with CMMG's "guard" barrel and BCG and am using Endomags. The thing runs like a sewing machine. Wish I'd never bought an MPX, and I would not trust it for a second for "serious" use, but would like to get it to run for 500 rounds or so between replacing springs so I or a buddy could at least shoot a match with it.



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This doesn't make sense and I'm feeling your pain. I wonder if it is possible your bolt is under gassed? Maybe the bolt is only going back enough to contact the ejector and the case rolls of the bolt instead of flying off. My load is a 147 Blue bullet making around 140 PF. But the thing I noticed is different is I would guess I launch brass at least 15 ft out and 5 ft forward. I am still on the factory recoil springs that have to have more than 10k rounds on them. Will your bolt lock back on an empty mag consistently? 

Edited by Chillywig
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Vary good possibly that the comp is increasing back presser. I would rebuild the extractor spring , remove the comp and shoot it to extractor spring failure .. willing to bet it will be more rounds than you have been seeing.


Hope you are using a new spring insert each time you change extractor springs,  not all spring inserts are created equal despite them being black I use these:



BTW I couldn't get an O ring or a defender to fit any of my 3 bolts

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3 hours ago, hhsmiley said:

Yes, I'm using a new insert each time. I'll try shooting it without the muzzle device and see.


I suspect I won't be able to get the  o-ring to fit either, Maybe with the ILWT extractor...

I have the ILWT extractors in all 3 bolts,   o-ring wont fit for me 

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Just as a contributing data point:


I removed my Taccom comp and replaced with a flash cone (because my barrel is shorter than my handguard). 

And the BCM extra power spring still got squished in about 15-200 rounds. 
I also really tried hard to fit the o-ring but couldn't get the pin in, so I left it out. 

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