Hi all.

I've got a buffer retaining pin which has twice failed, now, and am perplexed.

I've just finished building this rifle and it functions perfectly.  Very accurate and, after 400+ rounds, very reliable.  I'm having one problem, however.  The buffer retaining pin sheared off after about 100 rounds, initially.  So, I inspected everything and didn't see any damage whatsoever, replaced the pin ( I ordered 2 more fortunately) and went shooting.  This time I ran about 300 rounds through the rifle, again with no FTFeed or Fire, ejection is perfect, all in all a great rifle.  Today, as I inspected it, I open it up to find the pin sheared once again and that allows the buffer to spring out as I open it.  Also, this time the buffer looks like it's been whacking the pin hard: Here are two pics:

http://members.cox.net/romanpolaski/curiosandrelics/damaged%20buffer.jpg

http://members.cox.net/romanpolaski/curiosandrelics/damaged%20pin.jpg

Now I've got a Colt notched hammer and an AR slotted bolt carrier.  I've checked to see if there is any damage to the carrier and there is not.  It's a clean as a whistle. The buttstock assembly came as a unit from CDNN was all wrapped together.  It's brand new, too. The lower is an Eagle Armalite and everything fit perfectly. When it's inserted, the bolt carrier fits perfectly over the buffer retaining pin so it's not causing this problem. The buffer piston, however, is slamming into it as you can see by the photos.

It would almost appear that the buffer is returning too quickly, slamming into that buffer retaining pin much to hard.  

Also, if it makes a difference, I'm shooting only Federal 55 grain LC ammo...also clean and appropriate for this rifle.

I'm perplexed as to what to do about this. I can install my last remaining pin but am reluctant to do so until I can determine what's causeing the buffer to slam so hard into it.  Could the spring need to be adjusted??  Also, should I or can I polish up the buffer piston?

Any and all suggestions will be very helpful.

Rome

I did not see if this was a CAR buttstock or a standard A2, if CAR make sure you do not have a full size spring in.

If this is a car stock, make sure you do not have the buffer tube (extension) tightened to far to restrict the movement of the buffer retainer, I have seen these overtightened and cutting into the retainer setting up a failure point under stress.

Install new buffer and check for free movement with and without the spring.
Robert

Thanks for the suggestions, Robert.

This is an A1 buttstock. As I mentioned, it came as an entire assembly...all original parts so the spring is, I believe, the proper size as is the plunger.

I did check to see when the tube was screwed into the receiver, the buffer pin spring does move easily and is not being damaged by the tube.  And, when the whole assembly is fitted up, it fits perfectly with no obstructions and the pin moves freely when you push it down.  

I don't have any experince with these rifles to determine how much pressure there should be on that buffer plunger as provided by the spring.  It is seating properly, too.  At first, I did think that the spring wasn't seating all the way back into the tube but it is.  And, the rifle shoots and cycles perfectly.  But as you can see, that buffer plunger is really getting chewed up by the buffer pin.

Any other suggestions?

Rome

Please correct me if I am wrong, but as I understand it, the carrier/bolt should prevent the buffer from moving forward far enough to strike the pin.  Meaning, when the upper is closed (to the lower), the carrier should slightly press the buffer back, and the buffer should no longer bear on the pin.  Perhaps the pin is located too far aft, or it's an odd carrier/bolt/upper combination.  Hope you find a cure.

Stych

Stych hit it right on the head.  The receiver is out of spec with the buffer retaining pin location being too far back in the receiver.

Send the receiver back to have it replaced.

Wow.  I didn't expect to hear that. That's a pretty serious flaw if that turns out to be the case.

Can someone measure from the back of their receiver to the center of the pin whole? I'll use that dim to compare to what I've got.  Is there a spec sheet that lists the dims of the position of that hole where the pin hole should be located realtive to some known point like the rear of the receiver?  I'd like to measure this before I tear down the reciever and send it in.  That would address all of the problems I'm experienceing, however. I do appreciate your help here.

Rome

Stych & Dano are 100% correct.  

Another couple of things which can allow the buffer to hit the retaining pin is that (1.)the pivot & takedown pin holes are drilled too far forward in the receiver.  This also will move the entire upper forward with respect to the lower, or (2.)if the front face of the threaded portion of the upper where the barrel nut seats is too far forward you will have the same results.

Bottom line......either or both of your receivers are dimensionally out of spec .

OK  These are excellent answers but they lead me to a few more questions.  Bear with me, please.

1) If the upper barrel nut were out of spec, wouldn't the headspacing be incorrect as well?  I headspaced with the gauges and it specked out perfectly and it perform perfectly leading me to believe that it's fine. The brass bears that out.

2)The other issue that bothers me is that the buffer tube screws in perfectly into the lower receiver and apparently catches the detent pin, holding it properly but allowing it to move down and spring back up.  I would think that if the buffer detent hole was misplaced, either forward or backward, the buffer tube would either interfere with its operation or not catch it properly, allowing for the pin to escape.  

3) If the takedown pins were misaligned, the upper would not fit properly.  They do, however, when the pins are installed.

Can you address these comments?  Again, I'm in the extreme learning mode here, guys, and while I have no problem with sending back a lower for examination, I want to make sure I've covered my bases.

Rome

As far as the upper or barrel extention, if the outer shoulder of the extention were set too far aft, I believe that it could still be installed and HS'd correctly, and on the upper reciever, if the threaded extention for the barrel nut were too long, a correctly HS'd barrel asselbly would still be set too far forward.  In either condition, there may be other things that are about to run into one another, so I might double check the gas tube/carrier key too.  With the bolt fully in battery, does the carrier seat flush with the rear of the reciever?

I'm not real keen on the buffer tube assembly (luckily I haven't had to play with that yet), but yeah, as far as the holes for the pins go, I'd think that that would manifest itself clearly.  Keep us posted

Stych

I figured it out!  Not thinking it was important, I had ommited a detail which, as we can all imagine, is/was the cause.

First, of all, I remembered that my brother has a Bushmaster so I borrowed it and set it side by side with my rifle. Everything is identical including the bolts and the way they sit in the upper.  I also installed his upper onto my lower with no issues at all.

Then I reinstalled his upper onto his lower and noted that the rear of the bolt carrier contacts the face of the buffer plunger and actually pushes it back into the buffer tube ever so slightly. (the bolt is locked, btw). So, that's what keeps the buffer plunger from contacting the pin and shearing it off.

Then I placed his upper on my rifle and watched it close on the buffer plunger precisely as with the Bushsmaster lower. So far so good.

Then I put my upper on and watched carefully as it closed and noted that the rear of the bolt carrier barely touched the buffer plunger.  Ah HA.  That had to be it but why was it happening?

Well, it's actually quite simple. I have a colt, large hole upper on a small hole lower. I do have a special front pivot pin inserted which take up the slack with a sleeve but it's not enough. What is happening is that there's still some slack in there...probably 1/64"...and that's what I need to put the rear of the bolt carrier into the face of the  plunger to make it work properly.  I didn't notice this before because I've got one of those rubber "accuwedges" in the rear and it was artifically removing any slack that you'd normally notice. And, as I said earlier, I have little experience with this rifle.

The fix is straight forward. I'm going to have a smith fill and redill the front hole on my upper, placing the hole where it will push the lower back against the upper where it belongs, eliminating the bit of "slop". The rear pin hole is elongated in all uppers to allow for difference between all upper and lowers and I have plenty of space there.

So, this has answered my question.  

I appreciate your suggestions. They all made me think through every system on this rifle and now I'm pretty versed in what happens and what to look for in the future.  At least I know that the lower and upper are correctly spec'ed out and all I have to do is correct the front hole issue.

Thanks!  Hope this helps someone else in the future.

Rome

Kinky!

Glad you found the problem.  Cheers,

Stych

The buffer should never contact the retaining pin once the upper and lower are closed, the carrier should push back slightly on the buffer, you should be able to visually see this when closing the action slowly, look to see that the carrier is just barely pushing the buffer back, away from the retaining pin.  If it is not, let's think, could the retaining pin hole be too far rearward? (a machining mistake), could the carrier be too short?, could the cam pin hole in the bolt be incorrect, allowing the carrier to ride too far foward?  These would be very difficult to check without a blueprint a good measuring tools, but you could try changing out to different parts/ swapping with a friend or fellow shooter just to see if different parts will push the buffer back away from the retaining pin.  Just some thoughts that may help.  Of course, you could just leave the retaining pin out, but I know you want it right.

You're correct on all points.  As it turns out, however, the pin is perfectly located and the rest of the pieces are perfect. Only the front hole on the upper is the large version and the receiver is small.  I've remedied the problem temporarily by actually making a shim to the precise thickness I needed to take up the slack. Now the upper kisses the lower perfectly and the rear of the bolt carrier lightly detents the buffer plunger as it does with my brother's Bushmaster.  Cool.  I won't be damaging any more pins.

Thanks to you all for taking the time to help me figure this out. It was a challenge but worth it.

Rome

[size=3]DON'T DRILL OUT THE FRONT HOLE[/size=3]
You can get a front screw that is offset just to take care of this problem(bought one at the gunshow today)  OR you can get an insert that is offset that you can super glue in that you can turn whatever way you need to push your upper back a bit. Have one of these also, and it works. The screw costs about $9 and I think the bushing costs a couple of bucks, both will work.
HTH
Borg

What's got me confused is that the rear pin hole would have to be off too for the upper to sit too far forward. There is a bit of slack on the rear hole (it's not round) but I never thought there was that much slack.

ETA, I take that back, I have seen this happen with the offset screws. Not buffer damage but the upper sitting too far forward and binding when you try to close it.

Thanks, guys.

First off, don't be concerned, guys.  I'm NOT going to drill out the receiver.  I examined it and there simply wouldn't be enough meat around the front hole to be reliable. So, that's not an option. That leaves me with two choices:

Find a "shim" that will push the lower far enough forward to properly seat with the upper or "adjust" the upper front pivot pin hole so that I can have it redrilled properly.  

Someone at this forum said it best, however. Always "alter" the smallest and cheapest part first.  That makes perfect sense.  So, I did purchase two different front pivot pin/bolts with shims, etc.  I've played with them but, while they do remove most of the play, I'm still short a tiny amount.  I ordered both from Bushmaster, btw.

So, while the jury is still out as to whether or not I can use these successfully, I've fixed the problem using shims I made myself and affixed them temporarily with CA (Cyanoacrylate adhesive).  That resolved the clearance problem in the back and the rear of the bolt carrier does, indeed, kiss the buffer plunger now. I'm going to take it to the range today. If it fixes the problem, I'll come up with a permanent solution.

And, yes, that back locking pin hole on the upper allows for a lot of forward/rear movement.  I didn't even notice it before but it's quite elongated.

Rome  

How do you have the upper and lower attached to each other? I understand the lower is a small pin and the upper is a large pin. Are you using one of the press in bushings that bring the pivot pin hole in the upper down to .25"?

Yes, I have one of those. The "sleeve" that fits into the upper front hole is an eccentric with one side thicker than the opposite side. So, it's supposed to shrink down the upper front hole to the 1/4" standard pin size and move the pin forward in the hole.  However, the eccentric isn't really the exact size I need to make the fit proper.  I need for the pin to actually contact the front of the hole in the upper with all the slack taken up at the back of the hole. The sleeve I received does take up about 90% of the slack in back but not all of what I need for the "kiss" to occur.  The sleeve is round.  I need a "C" if you understand what I mean.

I also ordered another, different pin but this one turned out to be designed for a large hole lower and a small hole upper, the reverse of what I have.  I just kept it.  It had an eccentric built in, too, and would have done the job but I can't use it becuse it won't fit my lower receiver.

If you need clarification, I can attach a few photos.  

This isn't a big deal and with a little experimentation, I'll have this down permanetly soon.

Rome

That's what I thought, just wanted to make sure you had "adjusted" the bushing. Sounds like you need to get a piece of .375 Al round stock and drill a .25" hole in it then.

Sounds like a plan.  Thanks for your interest. This site has a lot of great info about this specific rifle.  It also must get a lot of traffic in the afternoon/evening as it's slow coming up but during the day, it's perfectly fine.

Thanks!
Rome

I would also recommend discontinuing use of the Accuwedge.  That piece of rubber in there likely exacerbated your problem by exerting pressure on the upper receiver in a forward direction, thus forcing the carrier even further away from the buffer.

Just my $0.02

Redleg, I have to agree. I'm not thrilled with it and it does make pulling the thing apart a PITA.  The fit and finish between the upper and lower are excellent and using the Accuwedge really doesn't thrill me so it's in the parts box right now.

Thanks for chiming in.

BTW, I'll take this opportunity to show you all a final photo which now shows the "proper" PG.
I'm very happy with this rifle and have learned a lot from it.

[img]http://members.cox.net/romanpolaski/curiosandrelics/AR15%20final%20photo%20left%20small.jpg[/img]

How about supergluing or J-B welding a 1/16" flat washer onto the buffer itself ? Do a little yankee gunsmithing .

Your rifle looks great, but in 1963 AR's and M16's had the 3-prong flash hider, not the birdcage type.  Change that out and you a perfect match!

That wouldn't allow the face of the buffer to sit forward of it's original location. Turning a shoulder into the circumference of the buffer face so that the buffer telescopes over the buffer detent does work. I have two buffers (rifle and carbine) in my tool box like this. I use them when I encounter rifles with this deficiency on the range.

That will shorten the throw on your buffer travel. If the buffer isn't contacting the rear of the carrier then that's what needs to be fixed. It's another halfas fix like tall front sight posts.

I am continually amazed in this text driven medium that so few people can read.

Hehe, calm down, it will be OK.

Yes I am amazed at some of the people you run into also. I post a link to a new product thats out there that I plan on using for a problem slightly different than the one posted. Then I get a smartas comment like this .

And a nice personal attack.

Guess better me than the dog.
Have a nice day. If you know how.