CMMG Guard binding issue

[TGibe]

I received my back ordered CMMG Guard 9mm lower today to match up with my CMMG upper ordered last month. Originally I thought I could use my Spartan lower with it but couldn’t get the bolt to clear the mag. 

 

Anyway, after snapping the two together tonight I couldn’t pull the bolt back more than halfway without it binding up hard. I took out the buffer and spring and everything looks ok (although the buffer rattles). The bolt goes into the buffer tube fine (spring removed) without resistance. 

 

I put the CMMG upper on my Spartan lower (after removing the ejector) and the bolt cycles fine (no mag of course) with no abnormal binding at all. If feels just like cycling the PSA bolt on the PSA upper. 

 

Could the buffer tube tube is misaligned? Could the upper be the problem even though it cycles fine on the Spartan lower?Has anyone else seen anything like this? 

Am I missing something I should try?

 

I plan to to call them tomorrow and am hoping for a quick resolution but wanted to gain as much info as possible. 

 

Just sucks having paid transfer fees etc, waiting a month and having it fail hard. LOL 

 

thanks,

Allen

 

[TGibe]

I even removed the trigger in case that was somehow interfering. It still binds. 

[bmiller]

When I talked to them about the bolt and barrel kit, they told me as long as everything is un-altered, their parts have a lifetime warranty.  I would imagine they will make it right.  Please let us know how this works out.

Edited December 28, 2017 by bmiller

[Aircooled6racer]

Hello: I would try the other upper on it to see if it will hand cycle. If not it could be that the buffer tube is bent or the machining in the lower is not correct. I am sure they will make it right. Thanks,ErIc

[Garmil]

When you are putting the cmmg upper on the cmmg lower did you try without the ejector in that configuration as well?

[TGibe]

2 hours ago, bmiller said:

When I talked to them about the bolt and barrel kit, they told me as long as everything is un-altered, their parts have a lifetime warranty.  I would imagine they will make it right.  Please let us know how this works out.

Thanks. I’m sure they’ll help me out and get me going. 

[TGibe]

1 hour ago, Aircooled6racer said:

Hello: I would try the other upper on it to see if it will hand cycle. If not it could be that the buffer tube is bent or the machining in the lower is not correct. I am sure they will make it right. Thanks,ErIc

My JB welded Taccom ramp interferes with their LRBHO linkage and the upper won’t close. Good idea though.

 

I thought about seeing if I could remove the linkage, but want to check with them first. Thx!

[TRUBL]

Yup......that ramp came out long before the guard......so it's never been tested with it. Not even sure that it is needed on the guard either. We do know that sometimes, the ramp can be modified however, to work with BHO features on some set ups......but for the most part, we state that it really doesn't work (out of the package) with BHO features.

[BartCarter]

10 hours ago, TGibe said:

My JB welded Taccom ramp interferes with their LRBHO linkage and the upper won’t close. Good idea though.

 

I thought about seeing if I could remove the linkage, but want to check with them first. Thx!

Maybe I am misunderstanding.  You have the CMMG Guard upper and lower and you put on a JB welded Taccom ramp?  How does that figure in your build?  The CMMG Guard upper/lower combination does not use a ramp.

[TGibe]

8 hours ago, TRUBL said:

Yup......that ramp came out long before the guard......so it's never been tested with it. Not even sure that it is needed on the guard either. We do know that sometimes, the ramp can be modified however, to work with BHO features on some set ups......but for the most part, we state that it really doesn't work (out of the package) with BHO features.

 

55 minutes ago, BartCarter said:

Maybe I am misunderstanding.  You have the CMMG Guard upper and lower and you put on a JB welded Taccom ramp?  How does that figure in your build?  The CMMG Guard upper/lower combination does not use a ramp.

I was only trying an upper from another complete PCC build to try and diagnose the issue with the complete CMMG guard build I'm working on. No ramp should be needed on the Guard.

 

Sorry for the confusion...

[TGibe]

12 hours ago, Garmil said:

When you are putting the cmmg upper on the cmmg lower did you try without the ejector in that configuration as well?

The guard doesn't use an ejector like a straight blowback. The ejector is part of the bolt.

[L9X25]

If the bolt moves freely in the upper, I would pull off the buffer tube and see if that solves the problem.

[TGibe]

Talked to CMMG and will send them pictures to look at, but the next step will be to return it to them unless they see something obvious. I couldn't see anything when comparing the buffer tube angles between the 2 lowers, so if it's off it isn't by much...

 

They said they will make it right, which I believe, but it's a bummer though as this is going to stretch well into January now...

[TGibe]

Sent pics last night and woke up to an RMA with shipping label from them. I called and talked to them and they think the buffer tube it out of alignment either due to the tube or the lower. They have a test fixture, but it seemed like they only do random checks so this one may have slipped through.

 

Shipped it off today and will wait and see what they find out.

[L9X25]

@TGibe

 

Do you have an update on what happened, or what they are did to fix it?

 

I have been thrilled with my NFA side charging upper, so I decided to build a matching NFA lower to let me put my JP back together.

 

I ordered all of the parts and everything seemingly went together fine.  Snap it all together and cycle the bolt ... and the bolt gets stuck in the buffer tube.  I apply some torque, as if trying to bend the rifle in half, and the bolt snaps closed.  If I hold torque on the buffer tube, everything cycles smoothly, but if I release the tension, the bolt binds and locks up.  It was too late for me to do more testing, but I intend to swap over the buffer tube from the JP and see if one tube is threaded crooked, or if the lower is threaded crooked.  

 

Any ideas?

 

 

[TGibe]

30 minutes ago, L9X25 said:

@TGibe

 

Do you have an update on what happened, or what they are did to fix it?

 

I have been thrilled with my NFA side charging upper, so I decided to build a matching NFA lower to let me put my JP back together.

 

I ordered all of the parts and everything seemingly went together fine.  Snap it all together and cycle the bolt ... and the bolt gets stuck in the buffer tube.  I apply some torque, as if trying to bend the rifle in half, and the bolt snaps closed.  If I hold torque on the buffer tube, everything cycles smoothly, but if I release the tension, the bolt binds and locks up.  It was too late for me to do more testing, but I intend to swap over the buffer tube from the JP and see if one tube is threaded crooked, or if the lower is threaded crooked.  

 

Any ideas?

 

 

 

I would remove and reinstall. Maybe try another tube if you have one. 

 

They said the buffer tube was misaligned and they removed it, reinstalled it, and test fired it. They said they don’t measure every lower on their jig. It is perfect now. I didn’t have tools and was concerned that the receiver was out of spec and relieved that it wasn’t. 

 

Now it need to work on a softer spring and lighter buffer so I can run 130-135 pf loads. Currently it needs 150+ to cycle fully. 

[L9X25]

I will R&R it and see if that makes a difference.  I doubt it will fix it, but who knows.

 

Thanks for the response.

 

[L9X25]

I loosened the castle nut and everything seems fine, although the threads fit too loose within the lower receiver.  

 

Took the JP rifle apart and the tube fits the lower receiver really tight.  Swapped the buffer tubes and the tube from the JP fits the NFA lower tight, as it did in the JP.  I tightened the castle nut and everything remained in alignment. 

 

I think the problem is simply a crappy buffer tube with over-cut threads.   It fits loose enough that it rattles in the lower receiver, even when the threads are at full engagement.  Tightening the castle nut locks it into a position, but not necessarily the correct alignment position.  I can see where I could hold it in alignment, while I tighten the nut, and it could work fine, but I doubt it would stay that way for long.  The solution is to order a good tube and trash this POS.  

 

Thanks again.