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JediGuy
07-31-23, 21:48
General Details:

FCD URF sans FA
CMT DRT 13”
Criterion Core 13.9”
BCM .625 gas block
Midlength gas tubes - Aero and Damage Industries

It’s been probably a couple years since I assembled an upper. But I’ve done more than a few, and I don’t recall noticing this issue before. Maybe I just didn’t notice.

Issue:
The Aero gas tube clearly makes contact with the CMT barrel nut. It does not “float” over it. I grabbed the Damage gas tube that I have as an extra, and it actually was now obviously making contact than the Aero was, with some pressure.

A little googling and a Criterion video point toward bending the gas tube as a valid resolution for this.

Anyone else bend their gas tubes? Any tricks to it?

I’m looking to get the most repeatable performance I can out of this upper, so I don’t mind doing it’s. Just new to me.
markm
07-31-23, 22:18
Iraqgunz used to use a large flat tip screwdriver.
556Cliff
07-31-23, 22:38
First, are you sure that your barrel nut is aligned perfectly? If not, get one of these. > https://www.specializedarmament.com/%E2%80%94-shop-tools/barrel-nut-alignment-gage-precision-ground/

Brownells has them too. They aren't as nice, but they work the same. > https://www.brownells.com/tools-cleaning/bore-barrel-tools/gun-barrel-tools/barrel-nut-alignment-gauge/

Otherwise, m-guns has a gas tube bender. I'd go that route over the screwdriver method. ;) > http://www.m-guns.com/tools.php
JediGuy
08-01-23, 04:52
Iraqgunz used to use a large flat tip screwdriver.

I am not visualizing that…


First, are you sure that your barrel nut is aligned perfectly?
…

Otherwise, m-guns has a gas tube bender. I'd go that route over the screwdriver method. ;) > http://www.m-guns.com/tools.php

The barrel nut isn’t timed, unlike a GI or some others. So impossible to be the issue.

I actually was looking at that Michigun bender last night! But it looks like it is for adjusting more within the receiver, if I’m understanding correctly. It appears that the issue with mine is further out.
ViniVidivici
08-01-23, 08:26
The flat blade would be for adjusting within the upper as well.

Personally, I don't think it'll be a problem as long as your carrier mates up well with the tube. I mean, if you give it a vend to get it off the nut, it will make it dip downward inside the upper, and then you'd have to bend THAT section back up, right?

I have no problem giving the tube a slight bend, by hand, if needed for good fit though.
556Cliff
08-01-23, 08:55
The barrel nut isn’t timed, unlike a GI or some others. So impossible to be the issue.

I actually was looking at that Michigun bender last night! But it looks like it is for adjusting more within the receiver, if I’m understanding correctly. It appears that the issue with mine is further out.

Ah, I get what the issue is now.

Have you tested to see if the gas tube binds up in the gas key?
markm
08-01-23, 08:57
The flat blade would be for adjusting within the upper as well.

This is what I meant. I assumed that the upper was complete, or he couldn't know for sure the final position of the tube.

Isn't alignment checked with an empty carrier and the upper half inverted? It's been a while for me.
JediGuy
08-01-23, 09:19
I was about to install the gas tube, and that’s when I noticed that with the gas block aligned and gas tube straight into the upper receiver, the bottom of the tube was impacting the barrel nut.
556Cliff
08-01-23, 11:12
Isn't alignment checked with an empty carrier and the upper half inverted? It's been a while for me.

That's the final check, to make sure the carrier freely slides all the way forward with little to no resistance and slides out under it's own weight when the upper assembly is tipped rearward.
556Cliff
08-01-23, 11:15
I was about to install the gas tube, and that’s when I noticed that with the gas block aligned and gas tube straight into the upper receiver, the bottom of the tube was impacting the barrel nut.

So is the barrel nut actually partially blocking the hole for the gas tube into the receiver?
JediGuy
08-02-23, 05:07
https://i.imgur.com/TpAymxb.jpg

https://i.imgur.com/bXiwr82.jpg

https://i.imgur.com/JEpWfgV.jpg

Here are some pictures of the original problem. If someone sees something obviously out of spec, I’d love to hear.

I messed around with a tube bender and found I do not (yet) have a light touch. “lol”

But it is better than how it started. Still some contact, but improved. I had to finish this so I can use it in an upcoming class, but I may purchase a BCM or Trajectory Arms gas tube after the class to see if there is a better fit without adjustments to the tube. As good as Core barrels are supposed to be, I would prefer “perfection” on this upper.
georgeib
08-02-23, 05:22
I don't see a problem in these pics. If you're able to slide the BCG in without issue, it's GTG.
ViniVidivici
08-02-23, 08:21
Right on dude, I see a can of Aeroshell on the bench.

I agree with George, to me that looks fine. As long as everything's kosher with the carrier and how it's contacting the tube, I'd just leave it.

It seems the design of this particular barrel nut is such that this amount of contact is inevitable.
556Cliff
08-02-23, 08:50
I can't see anything wrong either, other than the gas tube touching the barrel nut. I don't have any experience with that handguard so I can't say if it's a fluke or if it's that way with all the barrel nuts for that particular handguard.
DwayneZ
08-02-23, 13:09
It's a fully functional rifle? So the question is what?
JediGuy
08-02-23, 15:11
It's a fully functional rifle? So the question is what?

It’s a new upper.

My goal recently has been to maximize the accuracy potential, including minimizing any unnecessary contact points with the barrel.
DwayneZ
08-02-23, 17:59
It’s a new upper.

My goal recently has been to maximize the accuracy potential, including minimizing any unnecessary contact points with the barrel.

Perhaps that contact is helping to stablize the tube? I not sure the contact is causing any accuracy issue from your setup.

If the barrel nut is where it needs to be, index the nut to upper, also place a mark on the nut edge directly under the tube, take nut off and run a 3/16" or 5/32" round nose carbide down the nut, thus making a shallow groove to provide the no-touch you are looking for. Some type of setup on a mill is needed though. Re-install nut back to index. I assume you only need like maybe 10-20thou depth for the needed clearance? You can flat mill it, but that's taking off more than you need to.

If you want the clearance, not many options, etc. Do they make a smaller OD tube?