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MPX barrel wiggle?


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Hi!

 

I've got a second generation MPX. As an MPX, it has a removable barrel that indeed has to be removed to clean the gas system. Now here's the odd thing. With the two barrel bolts tightened to torque spec, after a few rounds the barrel becomes loose enough that I can noticeably wiggle it against the upper in all directions. Is that how it's meant to be, given that only friction holds the barrel in place (I never checked for this when the gun was new)? Precision is, shall we say, good enough for Steel Challenge. Any experiences from other MPXers? If it's not meant to be this way, any clues about the cause and the fix?

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The bolts don’t appear to loosen, but when torqued to spec they don’t seem to do much to hold the barrel extension in the first place (other than preventing the barrel from being pushed forward and out). 

 

I don’t know how it’s supposed to look since I have only one MPX, but what seems a little odd is that there’s visible air between the receiver and the barrel extension in the curved part between the barrel and the piston, where the bolts would seem to tighten up the receiver. I’m not even sure how this could possibly be a tight connection. 

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Here is a photo. Again, I'm not sure how exactly it's supposed to fit. But it seems odd that in the axis of the holddown bolts there's visible air between the barrel extension and the upper.

 

I don't think the bolts coming loose is the problem. Even with bolts freshly torqued to spec, I can notice some play, and the play becoming looser after a few rounds is probably just from knocking the barrel around a little. After that it doesn't get worse--ten rounds or two hundred doesn't seem to make a difference.

 

 

MPX.jpg

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10 minutes ago, Nathanb said:

Not normal. Is the gas block tight? 

From what I can tell, the gas block/barrel extension is rock solid on the barrel, but the entire assembly wiggles in the upper receiver. As in your case, the bolts don’t seem to make a big difference, except that in my case it’s loose either way. 

 

It sounds like this isn’t what it’s meant to be, and it’s time to talk to Sig. 

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I have heard of the shoulders of the bolts contacting the gas block before the head contacts the upper and it not letting the assembly tighten up. If that's the case, shims under the head can help. Or cutting the shoulder back.

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I don’t think your gas block is all the way in. It should be rotated to lock it in place during installation. My gen 2 also has one bridge with threaded holes for the bolts but the barrel doesn’t move at all.

 

Can you pull the barrel out and take a picture? I would like to see how the gas block is installed.

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14 hours ago, George16 said:

It should be rotated to lock it in place during installation.

 

Can you pull the barrel out and take a picture?

 

I'm not sure I understand. How does one rotate it, given the index pin holding it in one orientation? Here is the gas block. I also noticed that after a serious degreasing for the picture it sits a bit more firmly. (I normally just wipe it.)

 

MPX_1.thumb.jpg.4a285867430eb79a7358239c8cde33b1.jpg

 

9 hours ago, Gregg K said:

Don't know what is going on here but those 2 surfaces should be even with each other, is the gas block broken?

Here's a view from the top. The two front surfaces seems to reasonably aligned.

 

I still don't quote understand the principle of operation. Is the fit supposed to be so tight that it holds by itself, and the bolts merely prevent forward movement, i.e., unintentional disassembly? Or are those little bolts supposed somehow to compress that big receiver so that it becomes snug around the barrel? 

 

MPX_2.thumb.jpg.57fa65360b90bc4e818f964665ef617e.jpg

 

 

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Sig made a change in the diameter of the bolt heads at some point in the Gen II platform.  You should measure out the holes in the receiver where the screws are supposed to recess into then measure the diameter of the bolt heads that are trying to land in that position.  It is possible that your bolts may be the wrong diameter from an oops somewhere

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19 hours ago, chaosrob said:

Sig made a change in the diameter of the bolt heads at some point in the Gen II platform.  You should measure out the holes in the receiver where the screws are supposed to recess into then measure the diameter of the bolt heads that are trying to land in that position.  It is possible that your bolts may be the wrong diameter from an oops somewhere

 

Oh, I didnt' know that. The bolts are indeed ILWT replacements from when one of the originals snapped last August (despite the torque wrench). The holes in the gas block and the receiver appear to be 1/4", and the corresponding studs on the bolts and on the bridge with the two threads on the other side that receives the bolts (which is original), both seem to be nominally 1/4", too, an a tad thinner in reality. 

 

Looking at the length of the 1/4" studs on the bridge and of the corresponding section on the bolts, they hardly reach into the space in the receiver where the gas block goes, though. When I leave the receiver away and just close the bolts against the bridge around the gas block, that gives a reasonably tight connection. But that's apparently not what's supposed to happen, because that way the bolts stick out a little on the other side and there clearly wouldn't be enough space for the receiver left.

Edited by DoctorH
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The ILWT replacement screws don't work with the clamp nut "assembly" that has the two nuts on a plate.  I have both styles, the other being the individual nuts with flats.  I got my replacement parts from SIG direct.  ILWT screws work with the individual nuts with the flats style receivers.

 

Sig part numbers:

BARREL NUT 1810925-R  (this is the two nut "assembly")

SCREW, BARREL CLAMP 1810949-R

 

If you order parts from SIG, they will ask for the weapon's s/n

 

Hope this helps---my respectful contribution

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Update: My new original Sig bolts arrived, part 1810949-R. They are indeed different from the ILWT ones in that the wide part at the top is a little longer on the Sig part. I'll try out tomorrow what the barrel does when I put some rounds through it. Subjectively, the construction still feels a little flimsy, but we'll see.

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