Gas Tube Alignment

[Worn]

I just installed my 7.5" barrel, gas block and gas tube on my 300 BLK build (my first AR build ever). I was not happy with the gas tube/gas key alignment. I removed the bolt from the bolt carrier and ran the bolt carrier in and out by hand. It was obvious that the gas tube was not centered in the key and that there was significant friction with each closure of the BCG, as the gas tube bent slightly as it entered the gas key. Forward of the barrel nut, I could see the gas tube flex slightly as the BCG went forward.

I didn't expect perfect alignment, but it seemed to me that it was too far out of alignment. It probably would have functioned, but with accelerated gas tube wear likely.

I fiddled with it until the bolt carrier, when lowered slowly, with the muzzle down, would slide onto the gas tube with just its own weight. There was (is) still some small amount of friction.

How critical is this alignment? If alignment is off, is there some quick, proven way to make it right or is it going to be trial-and-error?

[smustian]

You will be fine with a minimal amount of interference. The tube will wear a little with use and will self limit.

[farm use]

Yea like smustian said your going to have some friction between the gas tube and key. As long as its not way off your going to be ok. You should be able to feel the tube enter the key with testing it without a bolt in the carrier. You should be able to adjust it and get it pretty close by adjust the barrel nut a little one way or the other. I have built quiet a few ARs and never had to bend a gas tube to get it right, just adjust the barrel nut.

[Worn]

Yea like smustian said your going to have some friction between the gas tube and key. As long as its not way off your going to be ok.

On a scale of "Perfect" to "Totally Botched", I'd say the friction I had upon first installing the gas tube was "way off" or higher.

You should be able to feel the tube enter the key ...

I don't know about "should". It seems like it's normal to feel some resistance as the tube enters the key. But if they were perfectly aligned and concentric, you would not feel friction. The dimensions appear that the O.D. of the tube's end is less than the I.D. of the key. Perfectly aligned, I don't think they would touch.

... You should be able to adjust it and get it pretty close by adjust the barrel nut a little one way or the other. I have built quiet a few ARs and never had to bend a gas tube to get it right, just adjust the barrel nut.

Wow. I would not have expected that. As a first-time builder, the barrel nut is the last thing I would think needed adjusting.

In my build, the hole in the barrel nut is perfectly in line with the receiver's hole. There is no interference at all inserting the gas tube. And fiddling with barrel nut torque seems a bit radical just to align a gas tube when all it needs is a bit of a bend.

I painted the end of the gas tube with fluorescent Sharpie, installed it, ran the bolt carrier in and out a few times, then pulled the gas tube and inspected it under black light to see where it was rubbing. Tweaked the gas tube accordingly. Lather, rinse, repeat. I used to use a plain black marker for such things but it's hard to distinguish the difference between a reflection from the black areas and actual exposed metal. So now I use fluorescent marker under black light. It's very obvious where the fluorescent has been rubbed away.

Perhaps a higher quality gas tube would have Just Worked.

In all the YouTube videos I've watched on building ARs, no one has ever mentioned gas tube alignment; they just install it and that's that. No one even checks alignment, much less adjusts it. In any case, this being my first AR build, I want it to be correct. It just seemed like something one should check. Glad I did.

[smustian]

The gas tube could have been bent slightly to begin with. Since you say it goes through the barrel nut without touching, just tweak it over a little at a time until you are happy then go shoot the snot out of it.

[farm use]

I do check my gas tube alignment on every build. You might want to try a different tube if that one isn't up to your spec. Most of the time the tube is just out of alignment from left to right because its binding between the upper and barrel but just a little and can be corrected with the nut. IV never had one that didn't rub or have a little resistance when going in the key. If you can get it to enter the key perfectly without touching would be great. But the tube has a flared flange one the end to help seal inside the key so its going to touch inside somewhere. That's just my opinion.

[plant.one]

Yea like smustian said your going to have some friction between the gas tube and key. As long as its not way off your going to be ok.

On a scale of "Perfect" to "Totally Botched", I'd say the friction I had upon first installing the gas tube was "way off" or higher.

You should be able to feel the tube enter the key ...

I don't know about "should". It seems like it's normal to feel some resistance as the tube enters the key. But if they were perfectly aligned and concentric, you would not feel friction. The dimensions appear that the O.D. of the tube's end is less than the I.D. of the key. Perfectly aligned, I don't think they would touch.

... You should be able to adjust it and get it pretty close by adjust the barrel nut a little one way or the other. I have built quiet a few ARs and never had to bend a gas tube to get it right, just adjust the barrel nut.

Wow. I would not have expected that. As a first-time builder, the barrel nut is the last thing I would think needed adjusting.

In my build, the hole in the barrel nut is perfectly in line with the receiver's hole. There is no interference at all inserting the gas tube. And fiddling with barrel nut torque seems a bit radical just to align a gas tube when all it needs is a bit of a bend.

I painted the end of the gas tube with fluorescent Sharpie, installed it, ran the bolt carrier in and out a few times, then pulled the gas tube and inspected it under black light to see where it was rubbing. Tweaked the gas tube accordingly. Lather, rinse, repeat. I used to use a plain black marker for such things but it's hard to distinguish the difference between a reflection from the black areas and actual exposed metal. So now I use fluorescent marker under black light. It's very obvious where the fluorescent has been rubbed away.

Perhaps a higher quality gas tube would have Just Worked.

In all the YouTube videos I've watched on building ARs, no one has ever mentioned gas tube alignment; they just install it and that's that. No one even checks alignment, much less adjusts it. In any case, this being my first AR build, I want it to be correct. It just seemed like something one should check. Glad I did.

barrel nut torque is a range, i dont think i've ever seen it be exactly one point ... ie: 55 ft/lbs. i've always assumed that it was because the barrel nut needs minor adjustment to get the alignment right for the gas tube. off the top of my head the range is somewhere from 35-80 ft/lbs.

i check alignment every time i put the barrel nut on. i have the wheeler vice block and gas tube alignment tool. if that gets through the grooves in the barrel nut without interference and your gas block is lined up right things usually work pretty nice as far as tube/key alignment goes, at least in my experiences.

[Worn]

I do check my gas tube alignment on every build.

As I noted, this is my first build and everything I know about the process comes from on-line sources, including YouTube. In the videos I've seen, no one even mentions gas tube alignment, much less checks it. But it seemed like something that should be done, especially since I was getting a "clunk" each time the gas tube entered the key as bolt carrier went forward.

You might want to try a different tube if that one isn't up to your spec.

I have just the one since I'm only building one AR pistol. Besides, it's easy to make adjustments.

Most of the time the tube is just out of alignment from left to right because its binding between the upper and barrel but just a little and can be corrected with the nut.

I would worry about the affect on barrel nut torque. While torque can vary widely, the initial torque is 30+ whatever it takes to get the gas tube hole aligned. Depending on which way the nut has to be adjusted to tweak alignment, the torque could go below the 30 ft-lbs minimum. I'd rather not chance it when a slight bend is so much easier.

IV never had one that didn't rub or have a little resistance when going in the key.

Yes, I suspect that to be the norm. Unfortunately, I had more than a little resistance and had nowhere seen it mentioned. So I did what I could.

If you can get it to enter the key perfectly without touching would be great.

And very rare, I suspect.

But the tube has a flared flange one the end to help seal inside the key so its going to touch inside somewhere.

And the more friction, the faster it would wear. I suspect that this would impair functioning, if the wear were extreme. Do people with many rounds downrange ever wear out a gas tube?

That's just my opinion.

Much appreciated. This is how I learn.

[dellet]

I would worry about the affect on barrel nut torque. While torque can vary widely, the initial torque is 30+ whatever it takes to get the gas tube hole aligned. Depending on which way the nut has to be adjusted to tweak alignment, the torque could go below the 30 ft-lbs minimum. I'd rather not chance it when a slight bend is so much easier.

Don't back it off to get the hole aligned, you do not want the torque to be below minimum. Advance to the next hole.

Ideally, set it to the minimum, check and then advance to the next hole and it should align before reaching the maximum torque. If you go to far, back the nut off and start over.

[Dr.Phil]

Just for clarification, here is the barrel nut tightening specifications:

CAUTION:
Do not torque over 80 ft-lb (108 N-m) while tightening barrel nut assembly to next hole, to allow for proper alignment of gas tube.
_________________________________________________________________________________________________________________________
Torque barrel nut assembly to 30 ft-lb (40.5 N-m) while maintaining sight alignment.
Barrel nut assembly may be tightened beyond 30 ft-lb (40.5 N-m) to align barrel nut assembly serrations for proper gas tube clearance.
Never loosen barrel nut assembly to align for gas tube clearance.
Check alignment of barrel nut assembly with upper receiver assembly.
The front 8 in. (20.32cm) of a gas tube may be used as an alignment tool.
This is inserted into the bolt carrier key and then inserted into rear of receiver.
If parts of barrel nut assembly are properly aligned, tool will pass freely and lay top dead center along the top of the barrel.
A number 15 twist drill (0.180in.) may also be used as an alignment tool.
If necessary, tighten barrel nut assembly to next hole to
allow proper alignment.

One other tip that I picked up from the USMC Depot Level Maintenance Manual is to torque to 30 ft-lb and then loosen the barrel nut three times prior to attempting alignment of the gas tube.