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Discussion Starter #1
Gun: MPX K
Part: mystery

I was foundling my MPXK last night, pulled back the charger and clink! this little thing came rolling down the empty mag well.

I'm about to take apart everything to see where is missing this "thing" but if anyone has any idea, please help point the way.

This is a very small part and it's metal. I picked it up with a magnet. 3/8" diameter. 1/4" thick.

On the inside there seem to be the cut out of a hex nut shape. The "white stuff" from one side seems to be some type a calcium-like material, not sure if its a build up of something (the gun has had about 500 rounds through it) or was there by design.
 

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I looked at MGW parts listing, then watched an MPX Disassembly video...and I used to own two MPXs...I have no idea. It does look like a Hex Head that was attached to something. My first WAG was part of the Gas Piston System...

Proceed with the autopsy and tell us what you find...
 

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Call Sig customer service tomorrow morning.
 

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Gotta agree with MoRivera. Looking closely at the last pic, you can see the "hex".

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9 would make sense. Someone or something could have over torqued the barrel bolts...looks like Mim material...Do you remove your Barrel to clean it?
 

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OP,
Is there any straight line knurling on the outside diameter of your pictured piece?

One of your allen head cap screws has sheared off the head leaving the stud in the threaded hole.

IF,you can not remove the stud w/ a pair of hemostats-

Take it to a professional to remove said screw stud as it is grade 8 material & will require a milling machine to remove it.
 

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It is probably the ejector plate screw. The ejector is right below the cam plate. SIG has redesigned the ejector on the stripped upper that I just built out. Those screws are a weak point, but SIG will fix it asap if it's under warranty.

Bill
 

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Discussion Starter #12
still a mystery...

Based on feedback so far, I took it apart to look for where that mysterious part or broken part belongs to. But still have no clue.

First photo is all parts from the strip down.
Second is my attempt to look for anything strange/missing/broken down the housing from where the barrel assembly sit.
Third is zooming in a bit of the second photo.
Fourth is the bolt.
Fifth is the bolt carrier left side.
Sixth is the bolt carrier bottom side.
Seventh is the bolt carrier right side.
Eighth is barrel+piston right side (the piston is shown on the bottom in the picture but when installed it would be on the topside of the gun).
Ninth is the top side view of the barrel+piston.
 

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Discussion Starter #13
barrel and piston assembly

More photos from the strip down.

The first here is the bottom of the barrel+piston assembly with the top of the photo being the front of the barrel.
The second photo is the view from the front of the barrel to the piston assembly.
 

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Discussion Starter #16
Confirming that the 2mm hex fits in there perfectly. But where did this come from!




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Is the broken hex head piece that fell out the same size as the hex bolts shown as #9 in the exploded view pic? Looking at all your recent pictures, everything looks fine, including the hex bolts in question.
Did you buy this new? Just curious if the previous owner broke one, replaced it and the old broken head was stuck inside somewhere til it decided to fall out.
I know, sounds kinda far fetched but everything looks fine in the pics.

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Discussion Starter #18
I’m the original owner. No the bolt size seems to be different than #9.


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As clean and unworn as everything looked, i figured you were the original owner. I wouldn't think that broken piece came from somewhere on the lower reciever, and you did say it fell out the magwell. Pretty crazy, you would think it would be something glaringly obvious to spot. I guess calling SIG would be the next step then. Sorry we haven't been much help

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