Faxon Gas Tube BS

[MaineMan2]

I am using a Faxon 16” Big Gunner profile barrel for a new upper. Faxon specs say “mid-length” gas tube. Aero Precision mid-length gas tube and M5 upper.  Gas tube measures 11-3/4” long. Dry fit barrel in upper receiver, line up gas tube hole with gas port on barrel, and I get this:

Looks like a 12” gas tube would be about perfect. Based on: 98’s thread the 11-3/4” gas tube should be correct but clearly it’s not. Using info from that thread, I did find that Armalite’s “carbine” gas tube is 12-1/16” long. Can’t find a “mid-length” tube anywhere I know of that specifies 12” length. Many are mystery parts, and the ones that do list length (like Aero Precision) all say 11-3/4”. Except Armalite itself.  Fulton Armory sells a “carbine” length gas tube - given that their “rifle” gas tube is 15-1/2” like Armalite it’s possible their “carbine” tube matches AR-10 specs as well. 
 

Should I go ahead and buy a 12” or 12-1/16” gas tube before moving ahead?  

[jtallen83]

You could try it and see, it's all about the balance between all the players, it's not gonna beat the rifle up or anything. If you have any issues I would say that would be top of the list to swap. Wilson Combat has an odd length gas tube or two.

[shooterrex]

https://dsgarms.com/upper-receiver-parts-arm10007005

[MaineMan2]

1 hour ago, shooterrex said:

https://dsgarms.com/upper-receiver-parts-arm10007005

Thank you!

[98Z5V]

7 hours ago, MaineMan2 said:

Looks like a 12” gas tube would be about perfect. Based on: 98’s thread the 11-3/4” gas tube should be correct but clearly it’s not. Using info from that thread, I did find that Armalite’s “carbine” gas tube is 12-1/16” long. Can’t find a “mid-length” tube anywhere I know of that specifies 12” length. Many are mystery parts, and the ones that do list length (like Aero Precision) all say 11-3/4”. Except Armalite itself.  Fulton Armory sells a “carbine” length gas tube - given that their “rifle” gas tube is 15-1/2” like Armalite it’s possible their “carbine” tube matches AR-10 specs as well. 

Inside that long diatribe of a rant that I made, and I think you're talking about my post of my rant on this - you'll remember that I went way off the rails about these companies that "drill in between" the spec of AR15 and Armalite AR-10 gas port differences, and just hope that people won't notice.  I went off about Faxon, specifically.  Faxon used to be a sponsor here, and Nate was the rep here, but they kinda disappeared here, before all this came out.  Too bad.  Would have had in inside source to gripe to over this bulshiit, but timing prevented that.  Seems Faxon still hasn't gotten it right. 

At any rate, I'm glad that you read what I posted, and picked up on what you need to do.  Armalite AR-10 Carbine gas tube, at 12 1/16" is your cure for the situation that you're in.  Glad you already found that out. 

[MaineMan2]

On 9/25/2022 at 7:12 PM, shooterrex said:

https://dsgarms.com/upper-receiver-parts-arm10007005

Thank you again. I was not familiar with DSG, but they were the only place I could find the correct Armalite ‘“carbine” gas tube at 12-1/16”.

Someone please tell Faxon this is how it SHOULD look!

 

Armalite part number is anyone needs it;

last digit is a “5”

I did check to make sure that this longer gas tube isn’t bottoming out in the gas key with the bolt closed.  It’s not even close - the gas tube could go at least 1/4” further into the gas key on the BCG. 

[shooterrex]

Fits just fine. Glad you found one.

[98Z5V]

MaineMan, that's perfect.  That pic right there means that Faxon drilled your barrel to the Armalite AR-10 standard.  That's exactly what it should look like. 

[FOGeologist]

What is the issue with the gas tube going to the middle of the cutout? What does that provide you with? The gas is getting released deeper into the key, but how would that influence the timing or pressures with these rifles, anyway?

FWIW, I bought a tube that allowed this configuration in my build, on advice given in the gas tube length thread.

[shooterrex]

The shorter gas tube doesn't push the BCG as long because it looses contact w/the BCG sooner. Not as much push=lower velocity.

[98Z5V]

11 hours ago, FOGeologist said:

What is the issue with the gas tube going to the middle of the cutout? What does that provide you with? The gas is getting released deeper into the key, but how would that influence the timing or pressures with these rifles, anyway?

FWIW, I bought a tube that allowed this configuration in my build, on advice given in the gas tube length thread.

Read this - it's been active for 4 years now, several years after all this started.  It'll help you gain some understanding.

 

[MaineMan2]

 The horse has been beaten post mortem; however, sometimes a video is worth a thousand words.  I typed up the following to get my own understanding squared away.  It may or may not be useful to anyone else.

At 4:40 in This video you can see the expanding gas entering the chamber inside the BCG. For a moment, the bolt is pushed forward by gas pressure at the same time the BCG is pushed backward. This reduces pressure on the bolt lugs while the cam pin is rotating the bolt out of battery. When the BCG has moved backward far enough for gas rings on the bolt to be forward of the vent holes in the side of the BCG, the gas pressure inside the BCG is released and the bolt can move backward with the BCG (under inertia now).  This reflects the genius of Eugene Stoner IMHO. It also demonstrates the critical timing involved in getting this system to operate correctly. 

Now look at the carrier key/gas tube connection during that cycle. If the gas tube were a bit too long, it’s OK. More gas will be released through the holes in the side of the BCG from that chamber inside it but the BCG and bolt can still keep moving backward. However, if the gas tube is too short, the gas seal between the carrier key and the gas tube could be lost BEFORE the BCG gains enough momentum to move backward as fast and as far as it needs to. (Please note that the gas tube could be so long that it bottoms out in the carrier key, preventing the bolt from fully locking into battery and that would be “bad”.)

Another video at 3:15 shows the gas system and bolt at work. (Turn sound down unless you like really bad heavy metal music) 

BTW that first video does an excellent job of showing how the trigger, hammer, disconnector and auto-sear operate.  The handoff of the hammer from the disconnector to the trigger always gave me the creeps but it makes complete sense for select fire.

 

[FOGeologist]

Thanks MaineMan2 and 98C5V, it's making better sense to me now.

I wonder why the AR15 doesn't have the nasty timing problem AR10s seem to have? I built a pistol a while back that was a bit of a shoulder-bruiser, until I put an Odin Works tunable GB on it.

I guess the four lengths are pretty well standardized due to military specifications? 

[MaineMan2]

2 hours ago, FOGeologist said:

Thanks MaineMan2 and 98C5V, it's making better sense to me now.

I wonder why the AR15 doesn't have the nasty timing problem AR10s seem to have? I built a pistol a while back that was a bit of a shoulder-bruiser, until I put an Odin Works tunable GB on it.

I guess the four lengths are pretty well standardized due to military specifications? 

Bingo!

[98Z5V]

13 hours ago, MaineMan2 said:

This reflects the genius of Eugene Stoner IMHO. It also demonstrates the critical timing involved in getting this system to operate correctly. 

This is exactly right, and exactly why I've been preaching this in the large-frames for so many years now.  Gas Timing.  This is a BALANCED effort, between the recoil system and the gas system. 

It doesn't matter what x-company or y-company produces.  Nobody cares.  Most of it is manufactured with profits in mind, foremost, making the platform as cheaply as possibly in order to make money.  That means shortcuts, on large-frames.  That comes in the recoil system, very first, to make it as cheaply as possible.  Doing it right is $$$, and tungsten ain't cheap.  Neither is stainless steel vs. aluminum, when you're looking at the body-material on a shortass 2.500" buffer. 

Then, they have to bastardize the gas system, in order to make a cheap recoil system "operate."  They usually "operate" like shiit.

First thing in one of these is "fully inspect and fix the recoil system."  After that, determine in your gas system is fucked up, then fix that.  Gas tube length is easy to check, and fix.  You need to get tools out, and be able to accurately measure gas port diameters - then fix it if necessary.