Two piece gas tube on AR upper???

[KennVFRidr]

Still learning as I go...so another question. Why would one use a two-piece gas tube???

After pulling the hand guard off what might be a Bear Creek .450 Bushmaster 16" upper, I noticed a faint line on the gas tube, it's hard to see...but definitely two pieces pressed together...

Any thoughts?

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[Koda]

Someone only "measured once and cut twice"...

Or BCA was trying to save money...

I'll let the expert builders chime in but that doesnt sound like an ok thing.

 

[Diamondback]

The ONLY legit reason I can see is if you're using a takedown kit like a Dolos or QRB. And those are purpose designed, not "weld two pieces of crap together and hope for the best."

Yet another reason for Bear Creek's somewhat dubious reputation...

 

[TTSX]

I see the line but I strongly doubt it's a two piece tube. Just a manufacturing blemish I'd bet

 

[tdemore1743x]

It almost looks like there's two, maybe a blemish from factory or from previous owner disassembling/assembling since it sounds like you got it used??

 

[Yarome]

I see the line but I strongly doubt it's a two piece tube. Just a manufacturing blemish I'd bet

Agreed. Likely just a tooling/machining mark. Even for a cheapo mfg... it would make utterly no sense whatsoever to incur the material, time and labor costs trying to mate two pieces of tubing together for a $5 part that likely only costs them a mere fraction of that.


I guess I "could" see some numbskull supplier in some 3rd world country suddenly finding themselves with hordes of rifle length tubes there isn't a very big market for and deciding to cut them down and shtick em together with a piece of bamboo and tree sap for shorter systems.....

It's not impossible.

 

[msgriff]

I might be missing something, but it looks normal to me. The Technical Data Package (TDP) for the M16 and M4 has drawings and specs for all components, including the gas tube, The drawings for the gas tube has a primary design and approved secondary/tertiary designs, as shown below. Note the differences in the diameter at the end and at the middle. Of course, civilian gas tubes don't need to follow these specs, but most do or use the spec as a basis for their products.

 

[KennVFRidr]

I see the line but I strongly doubt it's a two piece tube. Just a manufacturing blemish I'd bet

While I agree that two pieces seems improbable, it sure looks like it to me...???

 

[Koda]

While I agree that two pieces seems improbable, it sure looks like it to me...???

View attachment 1839314

its not clear. The only way a 2 piece would work is if they were swaged together to seal the gas.
Youd have to take the same closer detail pic from the other side/angle to see if the smaller diameter is inserted into the bigger.

But from here it looks like its just sized down like msgriffs blueprint spec shows.

 

[KennVFRidr]

It almost looks like there's two, maybe a blemish from factory or from previous owner disassembling/assembling since it sounds like you got it used??

View attachment 1839200

I'll call it second hand...a friend bought it new, set it in his safe, lost interest and never fired it. I snagged it from him and stored it until recently...it's never been fired by either of us.

I might be missing something, but it looks normal to me. The Technical Data Package (TDP) for the M16 and M4 has drawings and specs for all components, including the gas tube, The drawings for the gas tube has a primary design and approved secondary/tertiary designs, as shown below. Note the differences in the diameter at the end and at the middle. Of course, civilian gas tubes don't need to follow these specs, but most do or use the spec as a basis for their products.

View attachment 1839248

This looks like the answer...

Agree'd.

its not clear. The only way a 2 piece would work is if they were swaged together to seal the gas.
Youd have to take the same closer detail pic from the other side/angle to see if the smaller diameter is inserted into the bigger.

But from here it looks like its just sized down like msgriffs blueprint spec shows.

Just dug out a spare mid-length I have still new in the package from Stoner...and it has the same diameter change...just makes me wonder why...

 

[Koda]

and it has the same diameter change...just makes me wonder why...

its a good question.

Seems like there is no need but I did a quick check on Midway for several gas tubes and they all had this diameter change. Maybe it reduces barrel vibration where the gas tube inserts into the upper receiver (less contact).

 

[lucusloc]

I'll call it second hand...a friend bought it new, set it in his safe, lost interest and never fired it. I snagged it from him and stored it until recently...it's never been fired by either of us.



Agree'd.


Just dug out a spare mid-length I have still new in the package from Stoner...and it has the same diameter change...just makes me wonder why...

A lot of that is from the original hand-guard system. It is keept in the spec now for backwards compatibility and because it is unknown how many other aftermarket devices or components rely on it. This is one of the few disadvantages of the "lego" system in action; a whole lot of specs get added and are kept because no one knows the real impact of removing them. Sure it may be better to make a uniform tube, but not if it suddenly compromises the fit and function of an unknown number of other components.

 

[msgriff]

its a good question.

Seems like there is no need but I did a quick check on Midway for several gas tubes and they all had this diameter change. Maybe it reduces barrel vibration where the gas tube inserts into the upper receiver (less contact).

This is my understanding of why:
The middle section of the gas tube is 0.181" (-0.002") OD. The spec for the opening in the upper that the gas tube passes through is 0.182" (+0,003"). If they didn't turn down the OD at the end, it wouldn't be able to interface with the gas key, taking into account the tolerances of where the gas key/BCG presents itself, without exerting stress on the gas tube. Apparently, the 0..174" (-0.008") OD at the end provides enough movement so that all the stress isn't concentrated at the last ~1" of gas tube that is within the upper.

 

[Koda]

If they didn't turn down the OD at the end, it wouldn't be able to interface with the gas key,

except what I see on your print above is its not turned down all the way to the end.

 

[msgriff]

except what I see on your print above is its not turned down all the way to the end.

Correct, the bell at the end which interfaces the gas key has a reference diameter of 0.180". Of course, that portion of the gas tube never contacts the upper, except incidentally during installation. I'm talking about the area of the gas tube that is located within the gas tube opening in the upper, which provides clearance for movement.

 

[Koda]

Correct, the bell at the end which interfaces the gas key has a reference diameter of 0.180". Of course, that portion of the gas tube never contacts the upper, except incidentally during installation. I'm talking about the area of the gas tube that is located within the gas tube opening in the upper, which provides clearance for movement.

I think I get it now. So as the BCG moves into battery any misalignment with the gas tube allows it to flex home... ?

 

[msgriff]

I think I get it now. So as the BCG moves into battery any misalignment with the gas tube allows it to flex home... ?

Yes. I could calculate the total tolerance of where the center of the gas key mouth can be and still be in spec, but I would have to reference 3 or 4 separate component drawings. I would also have to take into account acceptable wear, which is in a different document set.

 

[monkeybutler]

Buy super cheap that is what you get.