Gas tube clicks against bolt carrier?

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Mar 23, 2003
When I hand cycle my midlength upper, I can see the gas tube relax just slightly when the carrier moves to the rear, and there's a light click when they mate up when the carrier goes forward. With the carrier out, I can depress the gas tube and feel it spring back a bit, whereas the gas tube is completely stiff in my rifle length upper (and there is no clicking sound). I've had no malfunction or failures yet, but is this a problem?
This shouldn't be a problem.

This is one area that some 99% of AR builders over-look or fail to understand.

So long as the gas key telescopes over the gas tube, it will work. Doesn't mean it is ideal though. It shouldn't slam into it causing a "tink" or "click" sound.

The reason for that is because the gas tube is not perfectly aligned. Just because the gas tube passes through the hole in the upper receiver doesn't mean it is lined up perfectly. The reason being is, either the gas block or the barrel nut is OFF alignment relative to the hole in the upper receiver. Either that or the gas tube itself is slightly bent. Like I said, the rifle can function fine, just isn't ideal. It doesn't take much at all to create that little "tink" sound. Tuning it perfectly is mostly nit-picking and difficult to get right.

I'm anal about these things, so mine are all tuned perfectly.

This is the test to determine whether or not you have proper gas tube alignmenet. AS PER the M16 Armorer's Manual:

Remove the bolt carrer and charging handle. Take out the bolt out of the carrier. Use just the carrier.

Now, insert the carrier into the upper while the upper is horizontal. Don't insert it all the way, only a few inches. NOW ....slowly tip the rear of the upper UPWARDS. This will cause the carrier to drop toward the gas-tube and chamber. The carrier's gas key should telescope over the gas tube when the entire upper is at 45 degrees or less. If it does that, then it is fine. It if requires being more upright, say 60 degrees, then it will probably still work - but is not ideal. If it has to go nearly vertical to drop over the gas tube - the gas tube is NOT aligned properly.

Whatever the case, the key will force the tube to center. It does that by slamming into it. You don't want the face of the key hitting the face of the tube and then bending it (even if the bend is very slight) into the key on every single shot.

Note, that it still might make a very faint "tink" sound when it telescopes over the gas tube even when at 45 degrees or less. That's ok. It won't be completely without sound. But it should be very faint. It shouldn't be a click or a louder "tink" type sound.

I hope this helps.
Oh yeah, forgot to add. A properly aligned gas tube will actually have a very tiny amount of "wobble" ...that means there's no force or bend being put onto it by a misaligned gas block or barrel nut. If the gas-block and barrel nut are in perfect alignment, and also aligned together perfectly relative to the whole in the upper receiver and the gas tube itself is not bent - then it will actually have a slight wobble to it. I mean a tiny bit. That's ok. That's actually perfect.

That indicates that there is no lateral pressure on the tube, which means there's no bend in it, which means everything is aligned just right.

If your gas tube is really stiff, there is side pressure on it. The vast majority of the time, a slightly over or under torqued barrel nut. No exaggeration, it literally just takes a hair one way or the other to do create that effect.

If it works and you have no problems - I wouldn't worry about it. If down the road you see the side of your gas tube getting worn badly because it rubs into the gas key with too much tension - then you might want to look into getting it corrected if the wear is severe or occurs early. I doubt it will.
Thanks for the detailed post. I did the test and the gas tube slides freely into the stripped carrier when the upper is only modestly inclined, much less than 45 degrees, so it looks like it's fine. I'll keep an eye on it as I put more rounds through it.
Then it is perfectly fine according to your results.

It's hard to tell what someone is describing on the internet. One person's "click" is very minor, another's may be very major. People have different ideas, sensitivities, observations and interpretations.

That's why I like objective type tests. I've seen AR's where the carrier would not slide down even when completely vertical. You'd have to barely touch it with your finger to close it. Didn't show when in the rifle due to buffer spring pushing it down. It's not good to add an extra obstacle to going into battery. The AR-15 doesn't have much mass or force going into battery. Not like an AK that has a long stroke and huge carrier mass that really slams the round home. You can see this when you put a fully loaded 30rd mag into the AR, and then hit the bolt release on a relatively new mag (or even just a regular mag). Doesn't lock nearly as fast as it does on the last few rounds. On it's own it won't cause a failure, but figure this. You have a full mag, a worn out buffer spring, a dirty receiver extension, a dirty cruddy action where the lube has dried up or burned up, a filthy chamber, the ammunition might not be slick and polished......ADD up all these factors, and the action is not closing as smoothly or as quickly as it should. Losing momentum, which can lead to a round not seating all the way. Not likely, but possible. You don't want a stiff gas tube on top of that adding more resistance.

A lot of factory built guns fail the 45 degree test.
Insert a #15 or #16 wire gage drill bit, or better yet, plain drill rod, into the carrier key.
Hand feed the bolt carrier into the receiver.
If the gas tube is properly aligned, the drill rod or bit will slide into the gas tube without effort.
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