Gunsmithing Reinstalling AR barrel extension

[308pirate]

I have no idea how this happened but today I found that my carbine's barrel was loose from the receiver extension. After zeroing some new optics, I went to check the zero on the irons and their cowitness. I look through the rear sight and center the Aimpoint's dot in the aperture only to find the front sight post is leaning to the right.

I grab the (pinned) FSB and literally can twist the barrel about 15 deg. What in the fuck is what I'm thinking. This rifle has never been dropped or abused. One high round count carbine class years ago but otherwise pretty easy life.

Anyway, I get home. Remove the free float rail (DD Omega so it comes apart in two pieces). Drive out the FSB pins and pop it fwd to get the gas tube out of the receiver. And then the barrel itself unscrews right out leaving the extension inside the upper.

Right now the barrel nut is soaking in WD penetrating oil. My Geissele barrel nut wrench won't make it budge, so I just ordered a Magpul wrench from Brownells.

The upper is 100% factory LMT, not some home build from a parts kit or some budget brand. I'm not even going to call them cause this upper is easily 10 - 12 years old.

Any idea what could possibly have led to a loose barrel/barrel extension joint? From what I've read around the net, that joint is supposed to be tightened to 140 - 150 ft lb, which is a shitload and should never come loose.

Can the extension be re-torqued on the barrel again and keep the indexing pin on the extension in line with the gas port on the barrel?

Or is this barrel shitcan material?

 

[padom]

If it's that old I would just grab a new Rainier Ultramatch or WOA or similar. Sounds like you got your money's worth out if it. But no I've never had a barrel come out of the extension.

Is the pin still in the barrel extension? You wont know until you get that barrel nut removed. Not only is the barrel extension torqued on, a pin is hammered into the barrel extension and into the barrel threads.

You could probably torque the extension back in then you need to repin it. I say just grab a new barrel.

 

[308pirate]

Are you sure the pin is supposed to connect the two? I've read elsewhere that it's not and my barrel's threads have zero indication that a pin ever made it down to them.

I am leaning towards a new barrel anyhow.

 

[chrismartin]

If you just screw the barrel back onto the extension (even when torqued correctly), the headspace will most likely be wrong and could be dangerous.
Even if you get things lined up (as you said with the index pin and gas port), you would still need to check headspace as well as it may not be anywhere near the correct torque at this point. It may never have been correct. I'd can it and get a match barrel or something (as suggested above)

 

[flyer]

First thing, call LMT.

They will probably be interested to see how it failed and might rebuild it for you or give you a new one.

Otherwise, I think the barrel is done unless you want to cut back the shoulder and add a shim to get the barrel extension timed and torqued right. It's probably cheaper to buy a nice and brand new barrel instead.

 

[flyer]

If you just screw the barrel back onto the extension (even when torqued correctly), the headspace will most likely be wrong and could be dangerous.
Even if you get things lined up (as you said with the index pin and gas port), you would still need to check headspace as well as it may not be anywhere near the correct torque at this point. It may never have been correct. I'd can it and get a match barrel or something (as suggested above)

If it's timed to the original location, headspace shouldn't change more than the barrel threads stretch (not much).

The other options would be 360° looser or tighter which would be off by one thread pitch and that's no good but cut the shoulder and add a shim properly and it should be good.

Of course, that's a lot of work to revive a well used barrel which has already gone sideways once.

 

[308pirate]

Thanks all, I'm going to call LMT so they are aware of the issue, and I'll order a new barrel with a pinned FSB.

Knowing what I know about how torquing threaded fasteners, I seriously doubt the BE will line up correctly once re-torqued to spec. I kinda don't trust this barrel any more.

Aero Precision has exactly what I want.

 

[padom]

Are you sure the pin is supposed to connect the two? I've read elsewhere that it's not and my barrel's threads have zero indication that a pin ever made it down to them.

I am leaning towards a new barrel anyhow.

No, I wouldnt say it "connects" the barrel to the extension. That's what torquing it on does. But I've seen threads on more than one occasion show impact from the pin.

 

[flyer]

I've never had to deal with a barrel extension but I always thought they torqued on the extension and then drilled the barrel pin hole and the gas port hole in the same setup or a jig that makes those holes parallel.

It would seem like the perfect time to drill past the minor radius of the barrel threads and make the pin lock the extension to the barrel.

I can't think of why they wouldn't do that unless it's something silly like they ordered barrel extension pins that were too short.

 

[308pirate]

No, I wouldnt say it "connects" the barrel to the extension. That's what torquing it on does. But I've seen threads on more than one occasion show impact from the pin.

Ah, got it. That makes more sense.

 

[308pirate]

Finally got that fucker off. The Geissele barrel nut wrench did it but it's a pain in the ass to keep from pushing the wrench too far in against the delta ring spring. That's why it was slipping off.

Will toss it in the junk drawer when the Magpul wrench gets here.

 

[308pirate]

Got the new barrel and a few odds and ends today including the Magpul wrench. That bitch is the shit! Made putting the new barrel on a piece of cake even when I had to torque the nut to get the next notch to align with the gas tube.

Nothing sucks more than cheap ass shit tools (Geissele barrel nut wrench) which aren't even worth what little you pay for them.