Brought this up on another board, nothing came of it, thought I would bring it up here.  At first I thought it was just chance I was seeing these, but am having second thoughts.

Have heard/read reports of "loose barrels" on M4 Carbines as early as 2001/2002 (specifically, USMC testing). Initially thought it was all bunk, but in the last few years I have seen some interesting stuff. Like barrel nuts that are not tight, and flop around the gas tube. On a friend of mines M4 upper there was enough room between the sides of the index pin that the barrel can be rotated a degree left and right. Really, I mean you could grab the FSB in one hand, and the upper receiver in the other, and twist it back and forth and feel slop in it. This has been described as a "loose barrel".

First thing out of the box  I checked the gas tube play at the barrel nut of my brand new M16A4/Colt's Model 901/905 upper with a born on date of 98. There was none. The nut was over indexed a few degrees too far so that a tooth of the nut was resting against the tube. The bolt carrier would pass over the tube with no resistance, but once it got withe an 1/8 of an inch or so of it's stop against the barrel extention it had a slight bind. Had to loosen the nut, and line it up correctly to get proper carrier key to gas tube mating. The barrel nut was not over torqued as in too tight, but simply not indexed properly at 12:00 TDC.

With the over torqued barrel nut the gas tube is in a bind and won't mate properly with the carrier key. This binding leads to malfunctions with the fired case staying in the action, failure to eject. This upper is from a 2000/2001 SFG issue M4A1 (RO921HB).  The barrel nut is timed a few degrees farther then TDC. The barrel nut is contacting the gas tube (I have had two USGI RO921HB uppers like this, and on one the nut was on WAY tight):



Only seen one of these, again another 2000/2001 SFG RO921HB upper.  On this one the nut was not index to TDC but stopped just short.  Again causing the gas key to bind on the gas tube:



The reason I bring this up is that I tinker with factory original USGI uppers that date as far back as 1963 and it has not been until the last few years that I have seen miss aligned barrel nuts with carrier key/gas tube bind. The oldest one so far I have found with bind had a barrel date of 1998. My experience with USGI XM16E1, M16, and M16A1 arsenal rebuilds is the same as pre-98 factory stuff, 100 percent correct carrier key gas tube alignment.  Three out of the four RO921HB upper I have had carrier bind, and as a result not capable of reliable function.  

Do you have an estimate on how tight these were put on?

I don't use a torque wrench, been doing this for 15 years by feel.  Have only had two Colt's barrel nuts that I felt were a bit tight, one was on a early 90's 6601 and one of those RO921HB uppers had a tight nut.  Far as loose nuts, only seen one, that being the one that you could turn by hand.

In other words, I have not observed a problem with the torque values, just that they are not timed to TDC.

ETA:

Oh yeah, one interesting observation though, each and every arsenal rebuild I have worked on the barrel nut is on WAY tight, I mean scary tight.

Taggity-tag-tag for the rebuttal.

Were these factory built or rebuilt by someone else

Also, were the two uppers you mentioned originally M4 uppers?  In the Air Force there's a good bit of "parts conservation."  They often re-re-rebuild rifles to get enough working weapons as possible, and replacing a barrel is not out of the question for any CATM shop.  Your examples are probably in the "close enough" category; the person who put them together saw that the barrel nut was "really, really close" or thought it was perfect because the BCG closed without too much trouble (though with any binding I think the free fall test would fail).

I actually not only eyeballed the one upper I built from scratch, but I used feeler gages to make sure that the same amount of space was on both sides of the gas tube, and an engineer's scale to make sure the tube was centered in the hole in the upper.  But I'm anal a stickler for details that way.

Very interesting observation Ekie. But After reading it twice, I am not sure what the conclusion is

In 15 years - a lot of barrel/upper inspections, you have seen 2 that seemed out of spec (to the experienced hand not the torque wrench) and one that was loose.

Is that a testimonial to the consistency of Colt manufacture/maintenance, or a slip in the details' since say 1998?

TIA

Quoted: Were these factory built or rebuilt by someone else

The M16A4 upper went from Colt's, to Dennis Todd, to me, as in factory original.  The M4A1 uppers are part of the batch that were sent out to SFG's in 2000/2001.  They came straight from the factory in these boxes:



The examples I have gotten are from Fort Bragg.

Quoted: Also, were the two uppers you mentioned originally M4 uppers?

Yes, as explained above.

Quoted: In the Air Force there's a good bit of "parts conservation."  They often re-re-rebuild rifles to get enough working weapons as possible, and replacing a barrel is not out of the question for any CATM shop.  Your examples are probably in the "close enough" category; the person who put them together saw that the barrel nut was "really, really close" or thought it was perfect because the BCG closed without too much trouble (though with any binding I think the free fall test would fail).

Yes, they fail the test.  Here is a correctly assembled RO921 from the first batch issued out in 1995:



Carrier goes all the way forward at this angle:



Here is a RO921HB upper from the second batch of M4A1 Carbines.  It will hold the carrier open even at this extreme angle.

I had a feeling this topic would go over like a fart in church.

It's still fun to watch though.

Quoted: Very interesting observation Ekie. But After reading it twice, I am not sure what the conclusion is In 15 years - a lot of barrel/upper inspections, you have seen 2 that seemed out of spec (to the experienced hand not the torque wrench) and one that was loose. Is that a testimonial to the consistency of Colt manufacture/maintenance, or a slip in the details' since say 1998? TIA

Yes, just a few barrel nuts that were torqued at a level I did not like, don't think that is an issue.

The issue I have is with the large percentage of barrel nuts not timed to TDC correctly since 1998.  Especially in the RO921HB's.  At first I thought that it was just chance that I was seeing this.  But now that I have had four RO921HB uppers and 75% of them have a gas tube bind that leads to malfunctions then I just got to ask, is this my luck or what?  If Colt's is having trouble with this then there are members here that  would know it.  

Quoted: Quoted: Quoted: Were these factory built or rebuilt by someone else The M16A4 upper went from Colt's, to Dennis Todd, to me, as in factory original.  The M4A1 uppers are part of the batch that were sent out to SFG's in 2000/2001.  They came straight from the factory in these boxes:... Quoted: ...though with any binding I think the free fall test would fail. Yes, they fail the test.  Here is a correctly assembled RO921 from the first batch issued out in 1995: img.photobucket.com/albums/0603/Ekie12091941/12196-firstbatchcorrect.jpg Carrier goes all the way forward at this angle: img.photobucket.com/albums/0603/Ekie12091941/12202-correctmating.jpg Here is a RO921HB upper from the second batch of M4A1 Carbines.  It will hold the carrier open even at this extreme angle. img.photobucket.com/albums/0603/Ekie12091941/12195-bind.jpg

Comparing the "bad" pictures in your first post to the "really good" picture of the first batch upper is very illuminating.  What looks like it could be just a bit of wiggle in the gas tube on the bad upper is really the barrel nut being off by just a tad.  But the good upper is primo-perfect-exactly dead on centered.  It's kind of hard to say one looks like the other, and it gives me the impression that someone was rushing to get the bad ones assembled "whatever it takes."

Further, applying my QC background, it looks like the bad ones never went past a second pair of eyes at the "ready for the handguards" stage of assembly.  A good inspector would have seen the cant of the nut and tube as plainly as if it had jumped out at him.  Bad QC means bad products go out the door, and rushing is never an excuse for bad QC.

This problem was discussed a couple years ago on ar15.com but I don't know how to find it via searching- lost to the internets I suppose-

Anyways- There isn't a lot to add- Apparently a decent # of the '00/'01 manufactured 'socom' hvy. barreled uppers weren't properly torqued as you've seen yourself- This problem was identified by the Army and Colt had to subsequently ship new, properly assembled uppers in ~2002/2003 to replace these.  Some of the documents about this are publicly available on a Army acquisitions website.  There was a ar15.com member who was an armorer who was able to give some of the back story on this- KevinB might remember something about this but I don't remember the login o fthe original poster.

My understanding is that there was a further change made to the barrel assembly itself in 2003 but I don't know what the nature of that was exactly.

Ken Elmore at SAW is probably the go-to guy to fill in details/correct anything I've mis-remembered..

Quoted: This problem was discussed a couple years ago on ar15.com but I don't know how to find it via searching- lost to the internets I suppose- Anyways- There isn't a lot to add- Apparently a decent # of the '00/'01 manufactured 'socom' hvy. barreled uppers weren't properly torqued as you've seen yourself- This problem was identified by the Army and Colt had to subsequently ship new, properly assembled uppers in ~2002/2003 to replace these.  Some of the documents about this are publicly available on a Army acquisitions website.  There was a ar15.com member who was an armorer who was able to give some of the back story on this- KevinB might remember something about this but I don't remember the login o fthe original poster. My understanding is that there was a further change made to the barrel assembly itself in 2003 but I don't know what the nature of that was exactly. Ken Elmore at SAW is probably the go-to guy to fill in details/correct anything I've mis-remembered..

That is the type of input I was looking for.  Now that you mention it, that story does sound familiar.  One problem I see with that story is for example the 10th SFG still has their 2000/2001 RO921HB's.  Another is, why not just have the armorers fix them?  Not that tough of a fix.

Never heard back from Ken on this subject.

Quoted: If Colt's is having trouble with this then there are members here that  would know it.

Honestly, Ekie, if you're looking for intelligent analysis of this issue, you've come to the wrong board as evidenced by the replies you've gotten already.  Sadly, the technical section of Arfcom ain't technical at all.  

Quoted: Quoted: Quoted: Quoted: Were these factory built or rebuilt by someone else The M16A4 upper went from Colt's, to Dennis Todd, to me, as in factory original.  The M4A1 uppers are part of the batch that were sent out to SFG's in 2000/2001.  They came straight from the factory in these boxes:... Quoted: ...though with any binding I think the free fall test would fail. Yes, they fail the test.  Here is a correctly assembled RO921 from the first batch issued out in 1995: img.photobucket.com/albums/0603/Ekie12091941/12196-firstbatchcorrect.jpg Carrier goes all the way forward at this angle: img.photobucket.com/albums/0603/Ekie12091941/12202-correctmating.jpg Here is a RO921HB upper from the second batch of M4A1 Carbines.  It will hold the carrier open even at this extreme angle. img.photobucket.com/albums/0603/Ekie12091941/12195-bind.jpg Comparing the "bad" pictures in your first post to the "really good" picture of the first batch upper is very illuminating.  What looks like it could be just a bit of wiggle in the gas tube on the bad upper is really the barrel nut being off by just a tad.  But the good upper is primo-perfect-exactly dead on centered.  It's kind of hard to say one looks like the other, and it gives me the impression that someone was rushing to get the bad ones assembled "whatever it takes." Further, applying my QC background, it looks like the bad ones never went past a second pair of eyes at the "ready for the handguards" stage of assembly.  A good inspector would have seen the cant of the nut and tube as plainly as if it had jumped out at him.  Bad QC means bad products go out the door, and rushing is never an excuse for bad QC.

More importantly it should have been found while conducting the cyclic rate test, or even while zeroing.

Quoted: Quoted: If Colt's is having trouble with this then there are members here that  would know it.   Honestly, Ekie, if you're looking for intelligent analysis of this issue, you've come to the wrong board as evidenced by the replies you've gotten already.  Sadly, the technical section of Arfcom ain't technical at all.

Already posted this over at 10-8forums, it was all new news to that crowd.

I find it odd that the nuts were not retimed and the uppers were in service for at least a year before being DX'd.

Elkie,

I got one of these Colt 2000 M4A1 uppers also that had a loose barrel. Would move back and forth in the upper. I took out the gas tube re tightened it and let it rip. Now using your "angle" method my carrier doesn't fall into place either. I give that to the bolt rotation resistance.

I have used this upper on my Colt M16A1 and did three "as fast as I can" F/A mag dumps with NO problems.


Just helping you out with my experience.

Quoted: Elkie, I got one of these Colt 2000 M4A1 uppers also that had a loose barrel. Would move back and forth in the upper. I took out the gas tube re tightened it and let it rip. Now using your "angle" method my carrier doesn't fall into place either. I give that to the bolt rotation resistance. I have used this upper on my Colt M16A1 and did three "as fast as I can" F/A mag dumps with NO problems. Just helping you out with my experience.

I just used the carrier in those pictures, no bolt, firing pin, etc.

Mine also runs like a champ, and has no problems doing multiple 30 round mag dumps, after I fixed it that is.

Quoted: I find it odd that the nuts were not retimed and the uppers were in service for at least a year before being DX'd.

Yes it is, and these have all been in the sand box.

Quoted: Quoted: Elkie, I got one of these Colt 2000 M4A1 uppers also that had a loose barrel. Would move back and forth in the upper. I took out the gas tube re tightened it and let it rip. Now using your "angle" method my carrier doesn't fall into place either. I give that to the bolt rotation resistance. I have used this upper on my Colt M16A1 and did three "as fast as I can" F/A mag dumps with NO problems. Just helping you out with my experience. I just used the carrier in those pictures, no bolt, firing pin, etc. Mine also runds like a champ, and has no problems doing multiple 30 round mag dumps, after I fixed it that is.

Ah!


Running to safe to do it your way..........................  BRB.


ETA  Doesn't bind up without the bolt. Sorry.

This is why I wish I could remember the name of the armorer guy- Hey could probably explain a lot of this- I would say that the ~3000 uppers that included the defective ones were shipped in lots of ~600 straight to each SFG and not all at the same time- its possible that the defect in question arose halfway throught the build so some groups got many defective uppers and others got none.

And I agree that it would be a straightforward fix for many armorers..but if Colt sends dozens of defective uppers to a SF group then they're (justifably) going to demand that they get what they originally paid for..

ETA- I believe the 'new' uppers were shipped with H2 buffers and the Colt gold extractor spring (vs. the chrome or black spring)

Quoted: This is why I wish I could remember the name of the armorer guy- Hey could probably explain a lot of this- I would say that the ~3000 uppers that included the defective ones were shipped in lots of ~600 straight to each SFG and not all at the same time- its possible that the defect in question arose halfway throught the build so some groups got many defective uppers and others got none. And I agree that it would be a straightforward fix for many armorers..but if Colt sends dozens of defective uppers to a SF group then they're (justifably) going to demand that they get what they originally paid for.. ETA- I believe the 'new' uppers were shipped with H2 buffers and the Colt gold extractor spring (vs. the chrome or black spring)

Was the armorer WpnsMan?

Quoted: I had a feeling this topic would go over like a fart in church. It's still fun to watch though.

ya i think so too, last time i commented on a colt here i got my ass chewed by a mod..........

Quoted: Quoted: This is why I wish I could remember the name of the armorer guy- Hey could probably explain a lot of this- I would say that the ~3000 uppers that included the defective ones were shipped in lots of ~600 straight to each SFG and not all at the same time- its possible that the defect in question arose halfway throught the build so some groups got many defective uppers and others got none. And I agree that it would be a straightforward fix for many armorers..but if Colt sends dozens of defective uppers to a SF group then they're (justifably) going to demand that they get what they originally paid for.. ETA- I believe the 'new' uppers were shipped with H2 buffers and the Colt gold extractor spring (vs. the chrome or black spring) Was the armorer WpnsMan?

I _think_ it was KMFDM but I'm not 100% sure.

wow!

I need a beer and a cigarette.


colt loses case against bushmaster and now this?


interesting

Elkie.

Here come the useless comments.


You knew it would happen.

Quoted: Quoted: Quoted: This is why I wish I could remember the name of the armorer guy- Hey could probably explain a lot of this- I would say that the ~3000 uppers that included the defective ones were shipped in lots of ~600 straight to each SFG and not all at the same time- its possible that the defect in question arose halfway throught the build so some groups got many defective uppers and others got none. And I agree that it would be a straightforward fix for many armorers..but if Colt sends dozens of defective uppers to a SF group then they're (justifably) going to demand that they get what they originally paid for.. ETA- I believe the 'new' uppers were shipped with H2 buffers and the Colt gold extractor spring (vs. the chrome or black spring) Was the armorer WpnsMan? I _think_ it was KMFDM but I'm not 100% sure.

Here is a post from KMFDM that may explain why these uppers were DX'd.

"If the Barrel is made before MAR 02 it is the older H.B. design and subject to a 10-15% (in my experience) failure rate for repeated failures to extract-and that is with the black spring and "O"Ring installed in any mode of fire (S/A, F/A, Burst) and it starts to happen worse after about 30-60 rounds fired. This one requires the "H" buffer. So be careful.

The Newer H.B. design after MAR 02 (Stamped on the front of the larger contour of the barrel, behind the forward handguard plate)has a different finish on the chamber walls (tighter Micro-inch surface spec.). It has less extraction resistance which stacks up in faster bolt cycling increasing the possibility for the bolt carrier rebound (aka: "bolt bounce") it operates on the tight end normally for the forward stroke movement of the bolt for cartridge pickup and bolt carrier rebound, not much margin for bad mags here. The "H-2" buffer is required for F/A and burst, rebound does happen occaisonally in semi-auto but not very often at all. If it comes with a bolt make sure the bolt has a "gold" spring.

Just cause I'm a Colt owner does not mean that I don't find this interesting and something to file away in the noggin. Info is info, good OR bad.

Could these have been at least temp fixed in the field by lightly bending the gas tube til they were looked at by a Armorer? I know this is not the proper way but would it have worked?

Damn!

Thanks Bradd!

I was going to go to a H2 buffer from H, but I have had zero problems. Now I see I have just what I need.

I also have the S.A.W. HD springs in my bolt.

Quoted: Quoted: Quoted: Quoted: This is why I wish I could remember the name of the armorer guy- Hey could probably explain a lot of this- I would say that the ~3000 uppers that included the defective ones were shipped in lots of ~600 straight to each SFG and not all at the same time- its possible that the defect in question arose halfway throught the build so some groups got many defective uppers and others got none. And I agree that it would be a straightforward fix for many armorers..but if Colt sends dozens of defective uppers to a SF group then they're (justifably) going to demand that they get what they originally paid for.. ETA- I believe the 'new' uppers were shipped with H2 buffers and the Colt gold extractor spring (vs. the chrome or black spring) Was the armorer WpnsMan? I _think_ it was KMFDM but I'm not 100% sure. Here is a post from KMFDM that may explain why these uppers were DX'd. "If the Barrel is made before MAR 02 it is the older H.B. design and subject to a 10-15% (in my experience) failure rate for repeated failures to extract-and that is with the black spring and "O"Ring installed in any mode of fire (S/A, F/A, Burst) and it starts to happen worse after about 30-60 rounds fired. This one requires the "H" buffer. So be careful. The Newer H.B. design after MAR 02 (Stamped on the front of the larger contour of the barrel, behind the forward handguard plate)has a different finish on the chamber walls (tighter Micro-inch surface spec.). It has less extraction resistance which stacks up in faster bolt cycling increasing the possibility for the bolt carrier rebound (aka: "bolt bounce") it operates on the tight end normally for the forward stroke movement of the bolt for cartridge pickup and bolt carrier rebound, not much margin for bad mags here. The "H-2" buffer is required for F/A and burst, rebound does happen occaisonally in semi-auto but not very often at all. If it comes with a bolt make sure the bolt has a "gold" spring.

^ this is excellent information for anyone.  I think colt makes a damn fine barrel and like to use them whenever possible.

Of course mine are government or M4 profile...I'm in the clear

originally from BraddD

Honestly, Ekie, if you're looking for intelligent analysis of this issue, you've come to the wrong board as evidenced by the replies you've gotten already. Sadly, the technical section of Arfcom ain't technical at all.

Why do you say that.. from the 2 pages I have read so far, the discussion seems to have stayed on track quite well with some good desemination of info.

OH MY GOD! not from Colt, do you mean that Colts can be imperfect? never heard of such a thing. I always thought that Colt firearms were made using magic from other forces from far far away and could never be flawed. will the madness ever end? but Colt quality control is far too superior to allow such things to happen like this is all i ever hear around here on the boards. guess the workers were drinking too much kool aid the day that these uppers were assembled, but of course most Colt cultist here will clam that the photos were photoshopped or that these are not "true"colt uppers assembled by colt workers or that someone sabotaged these uppers as part of the whole screw colt conspiracy. what ever the case may be these are truely sloppy assemblies from your true friends at colt.

what a concept, to think that people at colt are human and will and do screw up, no one is superior over the other when it comes to the top tier companies


now i will run for cover before the colt cult comes through and wages war on all of us non colt activists


camaro

Quoted: originally from BraddD Honestly, Ekie, if you're looking for intelligent analysis of this issue, you've come to the wrong board as evidenced by the replies you've gotten already. Sadly, the technical section of Arfcom ain't technical at all. Why do you say that.. from the 2 pages I have read so far, the discussion seems to have stayed on track quite well with some good desemination of info.

For reference, see the post between mine and yours...

Is this what caused the flood of used M4 uppers on to the market?

Quoted: Is this what caused the flood of used M4 uppers on to the market?

That's exactly what I was thinking...

Tagged

Quoted: Quoted: Quoted: Quoted: This is why I wish I could remember the name of the armorer guy- Hey could probably explain a lot of this- I would say that the ~3000 uppers that included the defective ones were shipped in lots of ~600 straight to each SFG and not all at the same time- its possible that the defect in question arose halfway throught the build so some groups got many defective uppers and others got none. And I agree that it would be a straightforward fix for many armorers..but if Colt sends dozens of defective uppers to a SF group then they're (justifably) going to demand that they get what they originally paid for.. ETA- I believe the 'new' uppers were shipped with H2 buffers and the Colt gold extractor spring (vs. the chrome or black spring) Was the armorer WpnsMan? I _think_ it was KMFDM but I'm not 100% sure. Here is a post from KMFDM that may explain why these uppers were DX'd. "If the Barrel is made before MAR 02 it is the older H.B. design and subject to a 10-15% (in my experience) failure rate for repeated failures to extract-and that is with the black spring and "O"Ring installed in any mode of fire (S/A, F/A, Burst) and it starts to happen worse after about 30-60 rounds fired. This one requires the "H" buffer. So be careful. The Newer H.B. design after MAR 02 (Stamped on the front of the larger contour of the barrel, behind the forward handguard plate)has a different finish on the chamber walls (tighter Micro-inch surface spec.). It has less extraction resistance which stacks up in faster bolt cycling increasing the possibility for the bolt carrier rebound (aka: "bolt bounce") it operates on the tight end normally for the forward stroke movement of the bolt for cartridge pickup and bolt carrier rebound, not much margin for bad mags here. The "H-2" buffer is required for F/A and burst, rebound does happen occaisonally in semi-auto but not very often at all. If it comes with a bolt make sure the bolt has a "gold" spring.

That is interesting, and does deal with the 2000/2001 batch, but does not address gas tube bind.  Unless perhaps gas tube bind was the actual problem but was misdiagnosed as something else?  Or it is simply that my sample size is too small to be statistically relevant?

For reference, see the post between mine and yours...

My bad.... I see your point......

Quoted: Quoted: Quoted: Quoted: This is why I wish I could remember the name of the armorer guy- Hey could probably explain a lot of this- I would say that the ~3000 uppers that included the defective ones were shipped in lots of ~600 straight to each SFG and not all at the same time- its possible that the defect in question arose halfway throught the build so some groups got many defective uppers and others got none. And I agree that it would be a straightforward fix for many armorers..but if Colt sends dozens of defective uppers to a SF group then they're (justifably) going to demand that they get what they originally paid for.. ETA- I believe the 'new' uppers were shipped with H2 buffers and the Colt gold extractor spring (vs. the chrome or black spring) Was the armorer WpnsMan? I _think_ it was KMFDM but I'm not 100% sure. Here is a post from KMFDM that may explain why these uppers were DX'd. "If the Barrel is made before MAR 02 it is the older H.B. design and subject to a 10-15% (in my experience) failure rate for repeated failures to extract-and that is with the black spring and "O"Ring installed in any mode of fire (S/A, F/A, Burst) and it starts to happen worse after about 30-60 rounds fired. This one requires the "H" buffer. So be careful. The Newer H.B. design after MAR 02 (Stamped on the front of the larger contour of the barrel, behind the forward handguard plate)has a different finish on the chamber walls (tighter Micro-inch surface spec.). It has less extraction resistance which stacks up in faster bolt cycling increasing the possibility for the bolt carrier rebound (aka: "bolt bounce") it operates on the tight end normally for the forward stroke movement of the bolt for cartridge pickup and bolt carrier rebound, not much margin for bad mags here. The "H-2" buffer is required for F/A and burst, rebound does happen occaisonally in semi-auto but not very often at all. If it comes with a bolt make sure the bolt has a "gold" spring.

Nice!  That would be the modification I heard about.  For all the crying around here this place can still be THE place to get the good info.

I noticed that my two LE 6920s are both timed like the one that is not TDC.  The carrier will still go fwd without much resistance on one, but on another it does hang up about 1/4 inch out of battery.  Both rifles still function 100%.  What will happen long term if the rifles are left like this?

Quoted: I noticed that my two LE 6920s are both timed like the one that is not TDC.  The carrier will still go fwd without much resistance on one, but on another it does hang up about 1/4 inch out of battery.  Both rifles still function 100%.  What will happen long term if the rifles are left like this?

You *might* cause inconsistent wear on the end of your gas tube.

and what will that cause long term?  How long would that take to be a problem?  Is it worth correcting?

Quoted: and what will that cause long term?  How long would that take to be a problem?  Is it worth correcting?

I'm far from being an expert, but I tweak mine just because I'm anal.  Hopefully, one of the guru's will be along shortly to be of more help.  The minimum that I can see is that you might have to replace a gas tube sooner while the worst I can see is that it causes your bolt and carrier to misalign while the bolt is trying to enter the barrel extension, but I'm not sure how much of a possibility that really is.