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 Help - Barrel extension pin came out!
dreiwhit  [Member]
1/2/2012 3:41:13 PM
I have a AR15 carbine that I bult about 20 years ago. Today I was swapping out an old A1 upper for a flattop upper, and I noticed that I could wiggle the barrel back and forth in the slot where the barrel extension pin goes in. So I removed the barrel and I was able to pull the barrel extension pin out with my fingers! I put in back in, tapped it ldown, but I noticed after I tightened up the barrel that the feed ramps appear to be slightly off center.

So, can that barrel extension pin be replaced with a new one, or should I be looking for a new barrel, or what is going on? I'm somewhat freaking out, since this is my SHTF carbine.

Whit
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Dano523  [Moderator]
1/3/2012 5:52:21 AM
The pin is just for general index of the barrel extension to the upper barrel socket.
So on that note, just punch the pin back in, and call it good.

When you tighten the barrel nut, go back and double check to bore sight against the sights. If need, take the barrel nut back off and shim the extension pin to the socket channel for a better zero, or just leave the barrel nut tight, and slip the entire barrel in the upper socket to correct the zero.

dreiwhit  [Member]
1/3/2012 6:27:37 AM
Thanks Dano! I was pretty worried, since I don't want to spend the money for a new barrel right now....

Do you think loctiting the pin into the extension would help keep it from wiggling?
shrikefan  [Team Member]
1/3/2012 6:38:44 AM
Originally Posted By dreiwhit:
Thanks Dano! I was pretty worried, since I don't want to spend the money for a new barrel right now....

Do you think loctiting the pin into the extension would help keep it from wiggling?


You can do what some manufacturers do and ever so lightly peen the edge of the hole in the barrel extension. Then lightly hammer the pin down into the hole using a lead, brass, or other soft metal hammer. If there is enough chamfer at the top of the pin you could probably get away with a ball peen hammer provided you didn't over peen the edge of the hole.

Gatorhunt  [Team Member]
1/3/2012 10:35:13 AM

Originally Posted By dreiwhit:
Thanks Dano! I was pretty worried, since I don't want to spend the money for a new barrel right now....

Do you think loctiting the pin into the extension would help keep it from wiggling?

Don't use loctite it will just ooze out when the extension heats up, you could try some rockset(sp) it's much more heat resistant.
Dano523  [Moderator]
1/3/2012 1:37:14 PM
It not like the pin is doing anything but a general index in the upper receiver barrel socket channel. Once the barrel nut is correctly tightened, it the tension of the barrel nut to the back side of the barrel extension that is holding the barrel in place.

So on that note, Try both ends of the pin to see if one is larger and has to be driven into the extension pin hole. If both sides are the same, then take one end of the pin, give a light tap with a ball peen hammer to slightly flare the end, then using the same hammer, tap it back down into the hole.

New install the barrel, bore sight to the sight, and if needed, adjust/slip the barrel in the upper barrel socket to index the sights line to the bore.

As for why the pin was knocked out of the channel, the barrel slipped in the upper receiver barrel socket when the barrel nut was being tightened, and slightly enlarge the pin channel. And yes, when the pin are being set, the hole is drilled slightly under size, and the pin just driven in to it so it stays.
shrikefan  [Team Member]
1/3/2012 1:57:05 PM
I guess some manufacturers are different but as I stated above, Armalite slightly shrinks the top of the hole on the extension to accomplish the task of holding the pin in.
dreiwhit  [Member]
1/3/2012 8:12:43 PM
Thanks everyone for their help!

I do have one question, only because i've never done it before and don't want to mess things up. When I go to peen the hole for the pin, what is the best way to peen it? I've got punches, and even a small ball peen hammer that I could use as a punch. I'm just trying to figure it out in my head in advance of giving it a try.
shrikefan  [Team Member]
1/3/2012 9:41:10 PM
Originally Posted By dreiwhit:
Thanks everyone for their help!

I do have one question, only because i've never done it before and don't want to mess things up. When I go to peen the hole for the pin, what is the best way to peen it? I've got punches, and even a small ball peen hammer that I could use as a punch. I'm just trying to figure it out in my head in advance of giving it a try.


Do you have some sort of metal ball bearing in the 1/2" to 3/4" range. Some of those magnetic toy kits would have one that would work if you have any kids. Just lay the ball over the hole and try to get a feel for it being lined up on center. Then just lightly tap the top of the ball. Check to see if the pin still slides in or if there is a little resistance. Repeat as needed...

Remember, the only purpose of the pin being snug in the hole is to help ease the assembly process. Once the barrel nut is tightened to the upper the pin CANNOT go anywhere.

Dano523  [Moderator]
1/3/2012 10:02:06 PM
Leave the barrel extension pin channel alone, and just peen the tip of the pin to make it slightly oval so it gets a bit into the extension pin channel as it being driven in to keep it from falling back out.

As for the feed ramps against the front take down lug, double check this with a dry fitting of the barrel to socket. If the barrel can turn in the socket (slop between the channel and pin), then you can shim the side of the pin against the channel via slivers cut from a coke can, to get the pin to slot tight with the correct barrel index before installing the barrel nut.


Also, depending on how you are tightening the barrel nut (upper receiver vise block, or barrel vise block), the chances of the barrel slipping in the sockets as the nut is being tightened differs (center of feed ramps off the center line of the front take down lug). So here, it better to us an upper vise block since the torque it just transferred directly to the upper receiver, and not like in the case of the barrel vise block, where the pin is taking the brunt of the force instead. Also to add, make sure that you are moly greasing not only the upper and barrel nut threads, but the front bearing face of the barrel extension since the barrel nut is going to friction up against front of the barrel extension as the nut is turning.

So we have the parts greased correctly, the upper in a upper vise block, and the pin shimmed so it's tight in the socket slot with the correct feed to lug index. Start by tightening the barrel nut to 30lbs, then back the nut off, go to 30 again, back it off, then on the last tightening, go to 35 lbs, check the alignment of the top nut spine void to upper receiver tube channel, and if needed, go more torque up to 80lb if needed to index the top barrel nut spine to the upper receiver gas hole. If you can not get the barrel nut to index correctly at up to 80lbs (don't go over 80lbs of torque), then back off the nut and try again. As the barrel nut is being torqued to the upper threads, it is mating the two together, and after a few tries, will allow you to index the nut without going over the 80lb torque.

Again, with the threads and barrel extension face to barrel nut inside contact surface correctly greased, do not go past 80lb of torque. Going past this 80lbs of torque is just going to end up with you either bending the upper socket (muzzle of the barrel with no longer be on the same plain as the center line of the receiver), or snapping the socket threads/cracking the socket from the body of the receiver!!!!!
dreiwhit  [Member]
1/7/2012 4:36:32 PM
I ordered a couple of new pins from Brownells. When they arrived today, I took my carbine out to the workshop and broke it down. My plan was to replace the pin with a new one. If that didn't work, tap on the new pin to slightly expand it and if that didn't work, peen the edges of the holes. If all that failed, it was new barrel time.

Fortunately, the new pin did the trick. I tapped it in with a brass hammer, and then put everything back together. Everything lined up and no barrel wiggle.

The only thing that I can think of was when I built this carbine 20 years ago, I didn't use an upper receiver vise (I don't remember them being available then.) Maybe I fudged up the pin then?

I want to thank everyone for their advise. Ya'll kept me from freaking out and kept me from having to spend $150+ on a new barrel.

Whit
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