I had just torqued down the barrel nut and was messing with the flashider when the barrel turned.  I thought maybe I did something wrong on the nut, but I realized the barrel was turning but nothing else. It unscrewed completly out!!

Does it have an index pin on top of the barrel extension?  I've never dealt with it but my understanding was that the pin passes through the extension and into the barrel to lock everything in place.

Quoted: Does it have an index pin on top of the barrel extension?  I've never dealt with it but my understanding was that the pin passes through the extension and into the barrel to lock everything in place.

the pin indexes, thats why they call it a indexing pin. It does not old anythign straight, thats why you tighten the Bbl nut, the friction keeps it on and not turning around, the pin is soft metal. If you did tighten the Bbl nut down, it wouldn't turn. If it did come out of the extension, it's toast and you need to send it back, you can't re-tighten the ext and have the headspace right

Got pictures? I'm having a hard time seeing what you're saying happened.

Thanks for the pics. I edited what I said, because now that I see it I realized I didn't know what I was talking about. You learn something new everyday. Sorry that happened.

It's all colt stuff, but it's used and not originally from the same gun.  The bbl ext is still under the delta ring. (pic 2)

So you cant use the same barrel extension twice. Thats interesting if you cant. I had a really reputable gunsmith do some work on a weapon and I know for a fact he didnt use  a new barrel extension. My indexing pin was sheared off and he ordered a new pin from Brownells and put it in the same extension.

Quoted: Quoted: Does it have an index pin on top of the barrel extension?  I've never dealt with it but my understanding was that the pin passes through the extension and into the barrel to lock everything in place. the pin indexes, thats why they call it a indexing pin. It does not old anythign straight, thats why you tighten the Bbl nut, the friction keeps it on and not turning around, the pin is soft metal. If you did tighten the Bbl nut down, it wouldn't turn. If it did come out of the extension, it's toast and you need to send it back, you can't re-tighten the ext and have the headspace right

I've assembled a few AR's, but I know verrry little about Barrel Ext's. I've never had to touch one.  I like to learn these things.  

Anyone know how they are installed in the first place?  What keeps them on, aren't they just torqued onto the barrel?  How is the Headspace set? or does this effect it?  
I definitely don't want to have a KB where the barrel & upper explode.

There's some good write-up's in the FAQ's, but I didn't find this.

thanks, LT

There's a special tool that fits into the lugs, sounded something like a socket, torqued to something like 100 ft lbs, IIRC; it just came up in a recent thread, but I can't locate it.

Try Midway, they should have it. I believe what he was saying was that you should send the CMMG barrel back if its busted.

Personally, I'd buy the 'socket' that fits the barrel extension if you already have a headspace kit and barrel vice blocks. If not... you're going to have to buy all three tools and you might just want to buy a new barrel with an extension.

If you have all the tools, remove the extension from the upper and re-torque it onto the barrel. If the sight still lines up wit the index pin (pray for luck) and it headspances properly when you re-install it on your upper... it will shoot fine and last a long time.

If the sight is not indexed and/or the headspace in incorrect... then I'd recommend buying a new barrel. At this point, you'll come out way ahead in money and time versus trying to fix your older barrel without the barrel extension.

Yeah, I bought it at a show last week.  It was clean and $100.  Buyer-be-ware I guess. Stuff like this is how KB's happen!!!

I thought I had every AR tool known. Anyway, after reading all this I'll just buy another barrel.  I was wanting to build an all Colt GI upper, just for grins.  

cheers, LT

BTW, I think the indexing hole/pin is there to make sure the feed ramps are in the correct position.

Quoted: Quoted: I've assembled a few AR's, but I know verrry little about Barrel Ext's. I've never had to touch one.  I like to learn these things.   Anyone know how they are installed in the first place?  What keeps them on, aren't they just torqued onto the barrel?  How is the Headspace set? or does this effect it?   I definitely don't want to have a KB where the barrel & upper explode. There's some good write-up's in the FAQ's, but I didn't find this. thanks, LT The barrel extensions are torqued on to 140 ft/lbs, so that they don't "just screw-off". And they are never supposed to be taken off, and should never "come off". You cannot restore it properly by just torquing it back on. Don't do that in an attempt to save money. They are torqued in place, and then the barrel is chambered and headspaced, then the index pin and gas port are drilled on the same jig to align them, and then the chrome lining is put in. It is a barrel assembly, that once the extension is put on, it's supposed to be on there forever. Either that extension had already been already removed by somebody, and then just screwed back on and sold to you(which would have been very wrong to do), or alot of unscrewing torque was applied by you during your assembling process to it to get it to unscrew(which would be abnormal since you wouldn't ever be applying torque in the reverse direction during assembly). They can be put back on by an experienced AR barrel maker, but it the chrome lining will be partially removed by the process, and it shortens the OAL of the barrel slightly when he re-sets a new shoulder on the barrel for the extension, so if it is a 16" barrel, it will be an NFA barrel after this is done. He basically has to "re-make" the chamber end of the barrel in this process, and it is time consuming and costly, and the result loses the chamber chrome and shortens the barrel slightly. It's just not worth it. The long and short of it is that the cheapest and best way out is to replace the barrel assembly with a new one. It will cost you more to try to fix this, and it will never be the same because the chrome lining will be fucked up in the chamber. Consider it a learning process and get a fresh barrel.

If he did hold the barrel in a vice ( not the upper )while torqueing the barrel nut on ,,HE IS applying torque in the reverse direction( unscrewing the extension )

BUT the extension should have been  torquet  at a higher rate than the barrel nut need to get ,,so the extension should not get of unless he did use to much force on the barrel nut

Quoted: So you cant use the same barrel extension twice. Thats interesting if you cant. I had a really reputable gunsmith do some work on a weapon and I know for a fact he didnt use  a new barrel extension. My indexing pin was sheared off and he ordered a new pin from Brownells and put it in the same extension. Quoted: Quoted: Does it have an index pin on top of the barrel extension?  I've never dealt with it but my understanding was that the pin passes through the extension and into the barrel to lock everything in place. the pin indexes, thats why they call it a indexing pin. It does not old anythign straight, thats why you tighten the Bbl nut, the friction keeps it on and not turning around, the pin is soft metal. If you did tighten the Bbl nut down, it wouldn't turn. If it did come out of the extension, it's toast and you need to send it back, you can't re-tighten the ext and have the headspace right

No you could use the same barrel extention twice, only thing is the headspace is set by special fixtures by how far the extention is screwed onto the barrel.

Quoted: Yeah, I bought it at a show last week.

So someone probably sold you their messed-up barrel.  You could try to retorque the extension onto the barrel, then check that it's centered, and headspace is correct (and good luck)... but for the cost of all the tools and the uncertainty of success you would probably be better off to just buy a new barrel.

All is not lost.  Do you have a headspace gauge?  Strip the bolt, removing extractor and ejector.  Screw the extension back on the barrel to index the pin with gas port.  If the bolt won't close on a NO GO gauge, but the extension is loose, you need to measure the clearance for a crush fit.  This is done by noting what fraction of a turn the extension can be screwed down.  That will be the thickness of a hard stainless steel shim needed.  Cut the shim to fit between the barrel shoulder and the extension.  Use a Dremel or similar with grinding wheels to dress it perfect on edges.

Now get the fragment of the index pin out of the barrel threads.  A broken screw extractor can be used.  If you cannot see it clearly, hit it with a wire brush, then with some cold bluing.  Use a drill press to drill the pilot hole.  

Install the extension with shim and clock extension to line up the holes.  You can use an OLD bolt or buy the tool from Brownells.  Or you can use a set of barrel/action vise blocks IF you can see the hole.   A bit of red locktite can be used if you torque is much less than 140 ft*lbs to prevent unscrewing BUT it probably won't hold up to high heat.  It will be a fine plinker or even match rifle.  Install a new pin (mild steel is fine, no brass) and it is GTG.

Maybe he should check with a good shop like ADCO or something to see how much they might charge for fixing it. It may be cheaper than a new barrel and it's worth looking into. I'm still getting parts together for my first AR, but I've done a lot of work on bolt guns...and generally leave critical stuff like installing barrels/headspacing to reputable smiths with expensive tools and a good reputation!

It seems like it's technically doable to get it fixed, but probably not a good idea for him to do it at home if he's never dealt with it before (not to mention having to spend a bunch on new tools that he may never need to use again.) I would imagine ADCO (or whoever he checks with) would probably tell him whether or not it's really worth fixing. Personally, if the price difference between getting it fixed or just buying a new barrel was only $20 or $30, I'd just get a new barrel.

Just my two cents (although they may not be worth that much!:-)

I would buy new barrel at this point.

Quoted: Quoted: Quoted: I've assembled a few AR's, but I know verrry little about Barrel Ext's. I've never had to touch one.  I like to learn these things.   Anyone know how they are installed in the first place?  What keeps them on, aren't they just torqued onto the barrel?  How is the Headspace set? or does this effect it?   I definitely don't want to have a KB where the barrel & upper explode. There's some good write-up's in the FAQ's, but I didn't find this. thanks, LT The barrel extensions are torqued on to 140 ft/lbs, so that they don't "just screw-off". And they are never supposed to be taken off, and should never "come off". You cannot restore it properly by just torquing it back on. Don't do that in an attempt to save money. They are torqued in place, and then the barrel is chambered and headspaced, then the index pin and gas port are drilled on the same jig to align them, and then the chrome lining is put in. It is a barrel assembly, that once the extension is put on, it's supposed to be on there forever. Either that extension had already been already removed by somebody, and then just screwed back on and sold to you(which would have been very wrong to do), or alot of unscrewing torque was applied by you during your assembling process to it to get it to unscrew(which would be abnormal since you wouldn't ever be applying torque in the reverse direction during assembly). They can be put back on by an experienced AR barrel maker, but it the chrome lining will be partially removed by the process, and it shortens the OAL of the barrel slightly when he re-sets a new shoulder on the barrel for the extension, so if it is a 16" barrel, it will be an NFA barrel after this is done. He basically has to "re-make" the chamber end of the barrel in this process, and it is time consuming and costly, and the result loses the chamber chrome and shortens the barrel slightly. It's just not worth it. The long and short of it is that the cheapest and best way out is to replace the barrel assembly with a new one. It will cost you more to try to fix this, and it will never be the same because the chrome lining will be fucked up in the chamber. Consider it a learning process and get a fresh barrel. If he did hold the barrel in a vice ( not the upper )while torqueing the barrel nut on ,,HE IS applying torque in the reverse direction( unscrewing the extension ) BUT the extension should have been  torquet  at a higher rate than the barrel nut need to get ,,so the extension should not get of unless he did use to much force on the barrel nut

We had a guy in the Oregon forum not too long ago successfully unscrew a Bushmaster Izzy brake that had been pinned and welded from the factory.  He completely fucked up his threads and I imagine he needed to throw the barrel away, but my point is that operation probably required a huge amount of torque, and his extension didn't unscrew.

This is turning out to be an "interesting" thread.  

You can certainly reuse a barrel extension, and this is your best bet to getting headspace and barrel timing correct.  If it were my barrel, I would simply tighten the extension back into place with red locktite to hold it.  Loctite 271 is good up to 200C or so for half stregnth - you are NOT going to get a barrel that hot.  I am using it now to hold barrel liners in centerfire pistols I've bored out - a 9mm and a 30 luger, and next is a 30 Mauser for a broomhandle.  

You don't have to have a magical 140 foot lbs for your rifle to function right - just make sure your front sight base is vertical.  Test headspace with factory new match cartridges if you don't have or want to rent gages.  This ain't rocket science.

Yes, I've had an extension come loose, but this was on my own build and I was reluctant to drill the indexing pin into the barrel threads.  Not anymore.    

I have never heard of fitting a barrel extension and then chrome lining the barrel.  What manufacturers do this and why, with sources?  That seems very odd to me considering how extensions are fitted by small shop or home smiths.  Normally, you face off the rear of the barrel, cut barrel threads, chamber it to a given depth, and then finish off the torque shoulder with a given barrel extension and bolt to set headspace.  That is it in a nutshell.  Simple as that, and it really chaps my ass for folks who know to obscure AR barrel chambering with an air of mistique.

I've never heard of a "special fixture" to set headspace, just a set of go/nogo gages and a depth micrometer and calipers.  That is all I use.  There are special homemade measuring tools that look intimidating, but they are essentially depth micrometers that operate faster and more precise.      

Instead of buying a 75 dollar barrel extension wrench, make your own.  I made my wrench out of two pieces of 0.5" x 1.5" 1018 barstock held together with two large bolts, with a 1.000" hole bored between the bolts.  I imagine you could do something similar with aluminum barstock and a 1" wood bit.

Facts and personal experience are welcome, but please hold back guesses.

Quoted: This is turning out to be an "interesting" thread. You can certainly reuse a barrel extension, and this is your best bet to getting headspace and barrel timing correct.  If it were my barrel, I would simply tighten the extension back into place with red locktite to hold it.  You don't have to have a magical 140 foot lbs for your rifle to function right - just make sure your front sight base is vertical.  This ain't rocket science. Yes, I've had an extension come loose, but this was on my own build and I was reluctant to drill the indexing pin into the barrel threads.  Not anymore.     I have never heard of fitting a barrel extension and then chrome lining the barrel.  What manufacturers do this and why, with sources?  That seems very odd to me considering how extensions are fitted by small shop or home smiths.  Normally, you face off the rear of the barrel, cut barrel threads, chamber it to a given depth, and then finish off the torque shoulder with a given barrel extension to set headspace.

+1

BIMHO this barrel is toast, for a home builder with general experience.

Quoted: This is turning out to be an "interesting" thread.   ... I have never heard of fitting a barrel extension and then chrome lining the barrel.  What manufacturers do this and why, with sources?  That seems very odd to me considering how extensions are fitted by small shop or home smiths.  Normally, you face off the rear of the barrel, cut barrel threads, chamber it to a given depth, and then finish off the torque shoulder with a given barrel extension and bolt to set headspace.  That is it in a nutshell.  Simple as that, and it really chaps my ass for folks who know to obscure AR barrel chambering with an air of mistique. ...

Why is it done that way?  For a chrome chamber.  Headspace is set before chrome is applied.  The original chrome plating process was feared to become contaminated due to swarf trapped in the threads.  And post-plating corrosion in the threads was a concern.  If you look at most chrome lined barrels, you can see the "mask" used on the extension as it isn't totally covered with chrome.  Some of the lugs will be chromed.  Hard chrome is NOT shiny like bumper/Harley chrome.  It is a matt finish.  But it effectively resists Parkerization so it stands out on a finished barrel.  Ditto at the muzzle.  

Brownell's has a barrel ext wrench (looks more like a socket) for $43 (member price). I also bought their "Colt mil-spec field guage".  So it looks like I'm going to learn more about AR's.  

Thanks for some great info guys.

cheers, LT

ps: I will see that dealer who sold me that barrel at a show this weekend.

UPDATE...I re-installed the barrel extension this weekend. I was thinking (oops) that since the thing was headspaced and alligned at one time, I should just have to tighten it down until the gas-port is in the 12o'clock position again. The indexing pin is mainly to make sure the feed ramps are in the correct position. I stuck a drill bit in the gas-port to have a good reference. It took a fair amount of torque and I did put alot of Locktight-271 on the threads. If I can get the indexing roll-pin out I'll drill the barrel a little (like pinning a flashider).

Anyway, it checks out with the headspace guages.  What am I supposed to measure with the depth micrometer? If all this works, then great. If not, I like to learn things (as long as it doesn't go KB).

cheers, LT

What did the dealer have to say?  If I knew how to contact him, I would have returned it.  He should have told you about that.

Quoted: UPDATE...I re-installed the barrel extension this weekend. I was thinking (oops) that since the thing was headspaced and alligned at one time, I should just have to tighten it down until the gas-port is in the 12o'clock position again. The indexing pin is mainly to make sure the feed ramps are in the correct position. I stuck a drill bit in the gas-port to have a good reference. It took a fair amount of torque and I did put alot of Locktight-271 on the threads. If I can get the indexing roll-pin out I'll drill the barrel a little (like pinning a flashider). Anyway, it checks out with the headspace guages.  What am I supposed to measure with the depth micrometer? If all this works, then great. If not, I like to learn things (as long as it doesn't go KB). cheers, LT

The depth micrometer is used to ream to final headspace.  It is not needed in your work.

I will assume you have both a go and no-go gauge.  As long as the no-go will not chamber on a naked bolt (remove extractor and ejector), all is good.  The AR15/M16 is one rifle that I would NOT recommend to use with a long chamber.  Other rifles have better case head support and if you reload, you can get away with reloads that do not have the shoulder set back as much.  But in the AR, such a case will probably blow-out due to the chamber mouth radius.

you could always trade it to two dogs,

Quoted: ps: I will see that dealer who sold me that barrel at a show this weekend.

Tag for what the dealer said...

Yea, I talked to him at the show. Didn't do much good. He swore that the barrel was fine when I bought it and that I must have done something wrong when I installed it. I was pretty mad that it could have had a serious malfunction (as in KB). I guess he can't admit anything for liability. I've installed at least a dozen barrels and never touched the extension. It was only hand tight, probably 20 ft-lbs.

Hey, 11b4v: You know two-dogs? I can derive from your name. Were you at the last "field trip"? Actually, I wasn't in attendence. I had female problems (as in; my female wouldn't let me go ).

cheers, LT

Quoted: UPDATE...I re-installed the barrel extension this weekend. I was thinking (oops) that since the thing was headspaced and alligned at one time, I should just have to tighten it down until the gas-port is in the 12o'clock position again. The indexing pin is mainly to make sure the feed ramps are in the correct position. I stuck a drill bit in the gas-port to have a good reference. It took a fair amount of torque and I did put alot of Locktight-271 on the threads. If I can get the indexing roll-pin out I'll drill the barrel a little (like pinning a flashider). Anyway, it checks out with the headspace guages.  What am I supposed to measure with the depth micrometer? If all this works, then great. If not, I like to learn things (as long as it doesn't go KB). cheers, LT

The pin should go through the extension and into the barrel.  Do it correctly with a drill press.  The old pin can be removed with Vise Grip pliers or equivalent tool.  It is just mild steel.  You will need a new one because it sounds like the old one sheared off or is too short.  Just drill deep enough to get full diameter below the threads, no deeper.

I guess it is too short. It is in place and is as tall as the notch in the upper-rcvr, but it never made a scratch in the barrel threads. This is all Colt parts though. They don't "pin" the extension on?  I'll do what you said this weekend.

cheers, LT