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Posted: 6/24/2012 8:35:21 AM EDT
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Shooting an Alex Arms Upper in 6.5 Grendel
and a Bushmaster Lower. After about 5 rounds- all of which feed fine, the BCA jams. After opening the rifle the Buffer retainer pin and its spring are found under the bolt. The buffer retainer spring is destroyed. So I obtained a new Wolf Buffer spring, new retainer pin and spring, new buffer, and new bufffer tube from Brownells. I tested the rifle and cycled the Bolt several times, and there was no problem. The Buffer retainer pin held its position under the edge of the Buffer tube which was thoroughly screwed in tight. So at the range today, the same thing happened after about 5 rounds. The buffer retainer pin popped out, the buffer pin retainer spring was jammed under the BCA. My loads are really not hot. Using Lapua Brass, 27.2 Gr of VV N140 and 123 gr 6.5mm Laupa bullet, and CCI primers. What is with the Buffer system and how do I fix it? hve gone through 2 sets of parts...Is it the BCA? Thanks. I know there is an expert here who can help out. |
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End of the tube should be kissing the center post of the buffer retaining pin.
Most of the time, a Carbine receiver extension will index without problem, while other times, the end of the tube has to be set back so the tube can be rotated another turn in on the receiver to get the end of the tube kissing the buffer retainer center pin. The fact that the spring is getting destroyed (loose in the lower receiver), and not just the center post of buffer retaining pin alone, this tells us that you do not have the tube spun in far enough in the lower receiver to correctly retain the buffer retaining pin, while still the tube is indexed correctly for the orientation on the stock. http://www.ar15.com/forums/t_3_66/577090_LMT_Receiver_Extension_install_questions.html |
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Checked the bolt carrier and the channel is smooth and cleanly cut
The upper closes on the lower assembly and as is shuld slightly pushes the buffer back so it does not rest on the buffer retaining pin. I have tried two buffer retainer pins one old and the other from Brownells that I put in this week. |
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Got new tube this wk
It is screwed all the way in, It holds the retainer pin in place with the receiver open. Checked the bolt carrier and there is no defect in the slot that rides over the buffer retainer pin. Cloxint the assembly demonstrates the buffer is pushed back enough that it does not hit the retainer pin. |
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Quoted:
Got new tube this wk It is screwed all the way in, It holds the retainer pin in place with the receiver open. Checked the bolt carrier and there is no defect in the slot that rides over the buffer retainer pin. Cloxint the assembly demonstrates the buffer is pushed back enough that it does not hit the retainer pin. If you're in the right spot, one additional (attempted) rev would cause the tube to bind on the pin which is undesirable, of course. There must have been tube damage or receiver damage for that retainer to escape ? Something's hitting it and the buffer is being properly held back, so it has to be part of the carrier making contact. What if you took some White-Out and painted the pin, then cycled the weapon by hand. Inspect the pin. Also observe how much "bolt is showing" at the back of the ejection port when fully retracted by hand. |
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Great idea with Wite Out
PAinted the pin. Retracted the bold manually and inspected the underside of the BCA and noted the slot in the underside cleared the pin . There was no resistance. When the bolt is retracted it is full y in the right place. My inspection of the buffer pin and tube are just like the posted picture above. Now what? |
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Quoted:
Great idea with Wite Out PAinted the pin. Retracted the bold manually and inspected the underside of the BCA and noted the slot in the underside cleared the pin . There was no resistance. So the White-Out remained intact.....no rub marks with the BCG fully retracted to its farthest point ? When the bolt is retracted it is full y in the right place.
Does the bolt disappear to the rear of the ejection port when being held back as far as it will travel ? My inspection of the buffer pin and tube are just like the posted picture above.
Now what? I'm still wondering how the retainer is escaping if the tube is threaded in to the correct distance. |
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Quoted:
Bolt appears all the way back when fully retracted. So the bolt is not visible in the ejection port when the BCG is fully retracted 100% of the way back ? If yes, one of the posters above eluded to reducing the rearward travel of the bolt. The theory is that maybe the forward section of the carrier is making contact with the retainer pin. You can show approx 1/8" - 3/16" of bolt in the ejection port and still have enough rearward travel for the bolt catch to work. To do this you have to spacer the buffer tube. You can use washers, quarters, what have you, to get the desired spacing. The spacers are placed under the rear end of the spring, then reinstall the spring/buffer system. Is it possible that this whole scenario was originally caused by the buffer tube not being threaded in the correct distance ? I can't see how the retainer and spring could have escaped without tube or receiver damage. |
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Very good advice so far Thanks.
My eyes ain't what they used to be..neither is anything else, but another time Under 3.5 X loupe magnification I note the hole in which the buffer ret. pin sits is not round. it is ovoid in the front and round in the back ..maybe from the pin slamming the front when the buffer retainer or the bolt catches it on the forward return? Anyway than might explain how it could be levered out of its socket. May have to build another lower with a better buffer retainer pin hole What do you think? Hard to tell which came forst at this point in time..a bad hole in the lower or damage from the buffer retaining pin being levered out on the return of the bolt to battery. |
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When the upper is opened, the back of the carrier comes off the face of the buffer, and the buffer lightly crashes forward into the buffer retaining pin. If the buffer retaining pin channel was milled too far forward to begin with, the very face edge of the buffer would be beat to hell, since it would be the buffer retainer holding the buffer in place, instead of the back of the carrier when the upper is closed.
Now since we do know that buffer when the upper is opened will lightly crash into the pin, so in a give time, the buffer retainer pin will oblong the channel slightly. But, we are back to square one, being if the receiver extension is worked so the face of the tube is kissing the center pin of the retainer (and it forward in it's channel) with it set correctly, then there is no reason to replace the lower receiver. To bottom line it here, you have lower receivers that have not been thread start indexed all the same, then even receiver extensions that are also wildly start point threaded as the start point of the thread to the tube. Short of if both the receiver and the extension are produced from the same manufacturer to hold this control, expecting that any receiver extension on a telescoping stock is just play and play to any lower receiver, is beyond wishful thinking. Here, in regards to such matters, about the only time you get plug and play receiver extension, is with a fixed stock receiver extension, since there is no butt stock orientation on the tube, and it just a mater of the face of the end of the tube, and the stop collar distance from it; with threading start point on the tube not mattering. So again, mod/shorten the end of the telescoping buffer tube so you can spin it in to do these four things. 1. it indexes the butt stock for the correct orientation. 2. the end of the tube kisses the center post of the buffer retainer with the buffer retainer all the way forward in it's channel. 3. Since the bottom face of the tube has been correctly timed for the two above items, make sure/double check to see if the top of the tube needs to be worked so it flushed with the inside face of the lower receiver barrel thread socket so not only can the upper/back of the carrier clear the end of the tube, but so the buffer front side bearing edges are always contained with in the tube. 4, And before you start doing the math of just how much the face of the tube needs to be faced, make sure that when the B/C and buffer are moved all the way rearward, the face of the bolts max travel backwards ends up with the face of the bolt from 1/4" to 3/8" in front of the back of the ejection port. So to take it back to the topic I listed, here should be your end results when you have installed the receiver extension correctly. Note, Op used a file, while I use a mill to end up with a smoother edge at the facing, but these is just a final appearance item, than a needed correctly fitting function item as he has acheived. 
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Pic Requested of Buffer pin and Lower buffert ube relationship
http://img545.imageshack.us/img545/7990/bufferpinwithwiteout.jpg |
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http://img545.imageshack.us/img545/7990/bufferpinwithwiteout.th.jpg
Maybe this is better |
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I don't want to speak for any of the guys that have been helping you out, but in case they are gone for the night, here's my take on that picture. It sure looks to me like the buffer tube is not screwed in far enough. It looks like the buffer itself is the only thing holding that buffer retaining pin in as it sits. You should be able to see the end of the buffer tube in that picture with the buffer inside it... not sticking way out past the end like that. you have to push that buffer retaining pin down and continue to screw that buffer tube in another turn or so... so that is is sitting on the lower shoulder portion of the retaining pin, just like the buffer itself is now in your picture. sorry if i'm not lokign at it right. :) |
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Quoted:
I don't want to speak for any of the guys that have been helping you out, but in case they are gone for the night, here's my take on that picture. It sure looks to me like the buffer tube is not screwed in far enough. It looks like the buffer itself is the only thing holding that buffer retaining pin in as it sits. You should be able to see the end of the buffer tube in that picture with the buffer inside it... not sticking way out past the end like that. you have to push that buffer retaining pin down and continue to screw that buffer tube in another turn or so... so that is is sitting on the lower shoulder portion of the retaining pin, just like the buffer itself is now in your picture. sorry if i'm not lokign at it right. :) agree with glg. It looks like if you were to push back that buffer the retainer and spring would come flying out. Thankfully the forward edge of the retainer bore still looks good. I've seen these broken out. Please remove the buffer and spring and show us the tube/pin relationship. Edit: I also see that your White-Out has been scraped off the top of the pin. This may be from contact with the carrier. 
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Quoted:
I don't want to speak for any of the guys that have been helping you out, but in case they are gone for the night, here's my take on that picture. It sure looks to me like the buffer tube is not screwed in far enough. It looks like the buffer itself is the only thing holding that buffer retaining pin in as it sits. You should be able to see the end of the buffer tube in that picture with the buffer inside it... not sticking way out past the end like that. you have to push that buffer retaining pin down and continue to screw that buffer tube in another turn or so... so that is is sitting on the lower shoulder portion of the retaining pin, just like the buffer itself is now in your picture. sorry if i'm not lokign at it right. :) Yep, receiver extension is not screwed into the receiver far enough to retain the buffer retaining pin. The end of the tube should be kissing the center post of the buffer bumper pin, and the end of the receiver extension over the back out flat of the pin. As for if the receiver extension indexes correctly or not, that is why I have included all the infromation needed to adjust/mod the end of the tube so it installs correctly in all the needed areas. |
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Quoted: Ok but now I have invested in a second buffer tube and honestly it is screwed in as far as it can go. That means "?" lapping the buffer lower junction to get it to screw further inward to push on the buffer pin? try and put up a picture showing more of the back end of that lower where the buffer tube threads in. if you took out the buffer, buffer spring, buffer retaining pin and buffer retaining pin spring, you should be able to screw that buffer tube in until it covers or least nearly covers that buffer retaining pin hole. the buffer tube is not threaded enough, has damaged threads or the lower has too much metal in the buffer tube hole area making the buffer tube socket too deep (too many threads). that would be where you said lap it , but it sounds and looks like more than my definition of lapping... like cutting off a sliver and then filing/sanding down the edge. I think we're on the same page anyway, but as for what to lap or file or grind off... better leave that for the other guys above that seem to know. I can only say that it ain't fitting together right! |
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Quoted:
Oh yeah an dlae today maybe I can get a picture up with the buffer removed and demonstrate the tube pin relationship. Would help, and the camera back far enough to tell what kind of stock you have in play. We are all guessing that this is a telescoping butt stock that can be threaded in far enough to solve the problem, but if a fixed stock receiver extension, then there is real problems going on here that is not going to be easily solved if the stop collar on the receiver extension is thread tight against the back of the lower receiver.. |
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Quoted:
Ok but now I have invested in a second buffer tube and honestly it is screwed in as far as it can go. That means "?" lapping the buffer lower junction to get it to screw further inward to push on the buffer pin? I'm going to go out on a limb and say from your pic that you are not differentiating between the retainer body and the retainer pin. It looks you are trying to bump the retainer body with the tube and not the actual pin. If that's correct, then that's the root of all your troubles. You must come in over the top of the retainer body with the tube and get as close to the pin as you can without binding it. |
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URL=http://imageshack.us/photo/my-images/217/004sfu.jpg/]http://img217.imageshack.us/img217/8373/004sfu.jpg[/URL]
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yeah if that's all the way in, then I think it's not going in quite far enough. coupled with the oval or elongated hole you mentioned up above, it sure looks like the pin may be able to slip by the end of that tube. espeicaly if it just takes a little notch out of the end of the tube on it's way to popping out. It's hard to say for sure tha't snot far enough, but in my experience, the buffer tube is able to screw in and cover the hole..... so you find the sweet spot and tighten down the locking ring. So, if that picture is "all the way in" then I'd say your question is now, what's stopping the buffer tube from going in far enough? |
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Quoted: Buffer tube stops when the shoulder of the tube hits the lower. yeah someone else will have to address that, I'm only familiar with buffer tubes that are the same size and threaded back a couple inches. Then there is a... basically a jam nut they call the locking ring that lets you tighten it down against the lower receiver when you have the buffer tube screwed in to your desired position. I can't speak to a buffer tube with fixed shoulder on it that contacts the lower reciever. (With what i'm talking about above the locking ring provides that shoulder, but the locking ring is threaded and can be spun forward or back on the buffer tube ) |
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Quoted:
Buffer tube stops when the shoulder of the tube hits the lower. Up until now we didn't know that you are working with a rifle tube. I thought that this was a carbine tube. You have something out of spec and it's probably the lower because apparently two different buffer tubes have yielded the same result. That's not enough coverage over the retainer. The grapevine says that CS from Bushmaster is lacking, but you could try contacting them anyway and see what they say. Or you could go to a carbine buttstock system that will allow you to manipulate the distance that the tube penetrates the receiver. The carbine tube does not have a stop on it like your rifle tube. It uses a castle nut for the stop. One other option is to run without a buffer retainer but you have to deal with the buffer and spring each time you open the receivers for cleaning. I suppose you could make up a dowel to serve as a temporary retainer while the receivers are open. It would be workable, but not ideal to be sure. |
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