MPX barrel wiggle?

[DoctorH]

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?

[Acer2428]

Are the nuts on the other side all the way in/engaged? But no, that's not normal. Maybe take a video?

[George16]

There shouldn’t be any movement at all. Do the bolts loosen up also when the barrel starts to wiggle? Did you put blue loctite on the threads?

 

 

[DoctorH]

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. 

[George16]

Can you post a pic?

[Rolex]

you need to use BLUE on the screws-

[DoctorH]

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.

 

 

[Nathanb]

Not normal. Is the gas block tight?  I could probably shoot mine without the pins secured and it won’t come out. And I’ve got two barrels 

[DoctorH]

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. 

[Chillywig]

Something is wrong here. Should not wiggle. POI should not change much after removing and reassembling 

[Chillywig]

Also your bolts look different than mine from than angle

[Nathanb]

You’ve got both sides of the screws right?  The nut does have a flat side that indexes and pulls tight

 

https://inleadwetrust.com/product/mpx-barrel-screws-and-nuts/

 

just want to cross all the bases 

[DoctorH]

42 minutes ago, Nathanb said:

You’ve got both sides of the screws right?  The nut does have a flat side that indexes and pulls tight

Mine has one bridge with threaded holes for the bolts, not separate nuts. 

[jlo86cj]

Your screw heads look like they should be deeper in. I have a two gen2's and they are more flush, like Chillywig's pic.

[jlo86cj]

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.

[NervousEnergy]

2 hours ago, DoctorH said:

Mine has one bridge with threaded holes for the bolts, not separate nuts. 

0_o

 

Is that an aftermarket thing?  Or a factory variant?  

 

Those bolts aren't pulling tight.

[George16]

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.

[Gregg K]

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

 

[DoctorH]

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.)

 

 

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? 

 

 

 

[chaosrob]

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

[DoctorH]

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 April 11, 2019 by DoctorH

[kattman]

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

[DoctorH]

That would seem to explain at least part of the problem (though somehow accuracy "good enough for Steel Challenge" isn't much different from what it was before....). I'll order new bolts.

[DoctorH]

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.

[DoctorH]

That seems to have done it. Thanks a million, @chaosrob!!!

 

(I still don't quite get how it's supposed to work. Are the bolts meant to headspace simultaneously at the heads with the upper and at the end of the wide part with the gas block?)

Edited April 25, 2019 by DoctorH