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Old November 25, 2024, 01:32 AM   #1
tangolima
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Gas tube slightly too long

It is an AR-10 (LR308 to be exact). The gas tube is exactly 1/4” too long, so that I can't close the action. I plan to put a lazy s-turn to shorten it. Any other suggestions? Thanks.

-TL

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Old November 25, 2024, 02:29 AM   #2
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Is this an assembly from a maker (like a complete upper?) or is it a Frankenstein you are building from parts??

For the first, contact the maker and get it replaced. If its sold new and non functional, its a defective product and the maker should repair/replace it.

If its something you're building from parts acquired from ??? different folks, you need to figure out which part is the wrong fit (most likely the gas tube, but possibly not) and then look at a getting a proper fitting replacement.

I'm not extremely familiar with the AR 10 but could you just cut off the end of the tube?? Or perhaps hone the inside of the bolt carrier key to allow the action to close??

Personally, I would get a different gas tube from some other supplier, and measure the one you have against the new one and see if the original is actually too long, or if, perhaps it might be the fit in the gas block, or the carrier key that makes it a problem.

If the action is stopping 1/4" from fully shut, (chamber empty) it would seem to me the most likely place to look is the fit of the end of the gas tube in the carrier key. A bit of work on one or both of those parts might give you satisfactory results without trying to do an "S" bend in the tube.

Hope this helps.
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Old November 25, 2024, 02:56 AM   #3
tangolima
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This one is a bit complicated. It is a custom barrel in 7mm SAUM with rifle+2" gas. Upper and BCG are cannibalized from other rifle. Without definite specs it is hard to pinpoint where the error is at. I reckon bending the gas tube is easier.

Tube end goes in gas key no problem. It just bottoms out in there. Can't just cut the tube. It will leak. The tube ends must be intact. I have no problem bending gas tube with tubing bender. I have done it lots of times, some of which were even done without proper bending tool. I do regard this benefit of DI. The gas tube can be formed to fit the other parts. I'm just making sure I haven't missed something obvious.

-TL

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Old November 25, 2024, 02:40 PM   #4
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Is it pinned in the gas block properly?
I know you're not an idiot and you've done this many times, but it sounds like the tube might be bottomed on the pin, rather than fully inserted.
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Old November 25, 2024, 02:50 PM   #5
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Tube end goes in gas key no problem. It just bottoms out in there. Can't just cut the tube. It will leak. The tube ends must be intact.
OK, but why not?? If you shorten the receiver end of the gas tube 1/4" it is still inside the key, right?? How is that going to "leak"?? And, if it does, how is that going to matter?? The key "leaks" anyway, venting into the action once it clears the end of the tube anyway.

Is the gas tube one of the ones with a "bulged" end to fit the key tightly, or is it just the straight tube? Would cutting the tube tip 0.25" shorter create a problem??

Maybe you should be cussin' the barrel maker for putting the gas port 1/4" too close to the receiver??

Bending the tube might be the best solution, for you, since you have some experience. For me, getting the right amount of bend on all four places would be a challenging trial and error thing, and I'd probably need several gas tubes to practice on before I found the right things to do.

Another possible option, is the gas block adjustable?? If so, might it be practical to remove the 1/4" from the front end of the tube, and redrilling the gas hole in the tube and the retaining pin hole so everything lines up properly again, and gives you the proper length so the action will close, and then just adjusting the gas block in use for reliable operation??

Just trying to think of different ways to skin this particular cat. Whichever one you choose to use is entirely up to you.
Good Luck! and do let us know which one you use, and how it works out.
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Old November 26, 2024, 12:21 AM   #6
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The gas block end of the gas tube is blind with 2 external grooves. If I cut it, gas will leak out of the now open end. The other end has a bulge, sort of like a gasket. If I cut it off, gas would leak out of now looser fit between the tube and gas key.

True that the gas system is not air tight. But excessive leakage will likely lead to malfunctions. It is not uncommon people fuzz around staking gas key bolts etc.

Redrilling the gas block doesn't seem viable. The gas orifice is already quite close to the edge of the part. I don't think I can possibly squeeze out 1/4”.

Sounds like that's it. I'm going reform the tube. But now the bender tool is nowhere to be found. I must have loaned it to somebody. Ordered one from Amazon.

-TL

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Old November 26, 2024, 01:47 AM   #7
Chuck Norris
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I thought I had that problem but I had the gas block in backwards,I flipped it around and all is good.
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Old November 26, 2024, 01:39 PM   #8
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I thought I had that problem but I had the gas block in backwards,I flipped it around and all is good.
I had not considered this, as all my experience has been with M16 pattern ARs, where the gas block (and front sight) can only go on, one way.

I'd say it is certainly worth checking to see if this might apply to your rifle.
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Old November 26, 2024, 02:55 PM   #9
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Originally Posted by 44 AMP View Post
I had not considered this, as all my experience has been with M16 pattern ARs, where the gas block (and front sight) can only go on, one way.



I'd say it is certainly worth checking to see if this might apply to your rifle.
I wish it is that simple. No luck this time. Everything lines up correct on the gas block.

-TL

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Old November 27, 2024, 04:16 AM   #10
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Ok I tried and failed. I managed to shorten the tube by about 1/8" before the turns became so big that it touched the free floating handguard.

The gas tube had total length of 17.125”, which is correct for a rifle + 2". I start to think it is the barrel. Emailing x-caliber for advice.

-TL

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Old November 27, 2024, 04:39 AM   #11
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My early model LR308 SASS features a non-standard "shorty" 308 carbine buffer, DPMS was known to use them; but still has no problems with standard bolts and carriers. Did you possibly swap out buffers? Assuming your gas tube is the proper length the only other possibilities are the gas block and/or barrel block shoulder/port are misaligned. Is the barrel dimpled to assist in clocking the block?
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Old November 27, 2024, 04:53 AM   #12
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When I build (Frankenbuild mix-n-match components) AR 10's I usually do a firing/cycle function check with the all the components minus the hand guard and assorted peripheral doo-dads, goo, locktite etc simply cause it's much easier for me to tweak that way than to continuously disassemble/reassemble the rifle in the treasure hunt to find the cause of issues.
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Old November 27, 2024, 06:34 AM   #13
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Just reread your post about where the tube meets the gas key--you say you've bent tubes lots of times--IMO the odds of you doing that and maintaining perfect alignment of the tube from gas port to key are very low--not to mention change the clearance of the tube between the barrel and handguard. If your tube is actually bottoming out against the back of the key--I believe you might have a length differential issue more than a 1/4."
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Old November 27, 2024, 12:03 PM   #14
tangolima
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I figured it out. The problem is actually in the other rifle, where I move the BCG from.

I found relevant specs online to check the gas tube and barrel dimensions. There were all correct. The suspect is now the BCG. I previously inspected the gas key. There is a step in there with close to perfect profile. Poked on it. It was hard as metal, and that was what stopped the action from closing. Could it be machined erroneously? Why there hadn't been problem in the other rifle?

I tried hand reaming it with a drill bit. A big pile of carbon powder fell out. Ah ha. The gas tube in the other rifle is actually exactly 1/4” shorter then normal. Carbon has built up there to take the void. Funny is that the gas passage is round and centered, as if it is machined. I think the gas tube was shorter to avoid trouble with non-standard nature of ar-10/lr308. Works but not nice. Lesson learned.

Now it is all good. Now I need to focus on loading ammo. Just received the free bullets from Hornady. Will go through the ritual to check bullet jump etc.

Before bending gas tube (last resort), I put the tube on big piece of paper to trace out its profile. It is the template. What matters is the few inches on both ends. They need to align. The bender is tricky to use, more so with the cheaper versions. The jaws slip, making the bend imprecise. I need to supplement with visegrip and lube. Not professionally perfect, but it works ok. You check the alignment with closing the action manually, with and without the gas tube installed. There shouldn't be noticeable rubbing. Usually some minor tweaking will make it right.

All good. Thanks guys.

-TL

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Old November 27, 2024, 04:22 PM   #15
tangolima
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Dug into the story further and found some clues. Looks like it is another armalite versus other guys issue. Gas port location on barrel has 2 versions. Armalite drills it 1/4” farther towards the muzzle than the others. Apparently the other rifle of mine follows the armalite pattern. With standard 15.125” rifle gas tube, it would be 1/4” short but still functioning, plus the carbon build-up.

To be kosher, I can order a correct gas tube from armalite or white oak. But I'm not going to. I have the drill bit.

-TL

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Old November 28, 2024, 02:19 AM   #16
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The stuff you get yourself into...I have to admit it's some of the most entertaining material on this forum.
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Old November 28, 2024, 02:31 AM   #17
tangolima
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Yup. Pulled the other rifle's barrel to measure the gas port location. It is 13.75” from edge of the barrel extension. It should be 15.5", so it is indeed a armalite spec. It is the .243 win barrel I have been playing around. Funny eh?

Quite unintentional I may have turned myself into an expert of the useless kind.

-TL

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Old November 28, 2024, 03:46 PM   #18
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So, are you going to straighten the tube you bent?? Or leave as is?
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Old November 28, 2024, 04:22 PM   #19
tangolima
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I straightened it. With the original profile traced out as template, it is pretty easy.

-TL

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Last edited by tangolima; November 28, 2024 at 05:07 PM.
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Old December 9, 2024, 10:25 PM   #20
tangolima
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Took the rifle out for test fire this afternoon. It worked fine after opening up gas by one full turn. The ejection port will need to be opened up more. Ejection is a bit iffy.

139gr bullet going over 3000fps is impressive, at least to me. Surprisingly there was almost no felt recoil. Between the big hairy muzzle brake and the sheer weight, it kicked softer than a 5.56. Group was preliminary but showed potential.

-TL

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