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View Full Version : new AR build, bolt catch issue


AmbientR
05-30-2011, 11:50 PM
I just completed my first AR build. Its a complete stag arms upper over an Ammo Bros defcon stripped lower with daniel defense parts kit, ATI buffer, and magpul moe stock.

Overall I am very happy with the rifle, it looks great and shoots like a dream. But there is one issue.. It seems like the bolt does not pull back far enough to get caught by the bolt catch. This means that after I fire my last round in the mag, the bolt won't stay open. Also, using the charging handle, I still cant get it to pull back far enough to manually engage the bolt catch. The rifle cycles rounds properly though, and is otherwise completely reliable so far.

I am rather certain it is because of something in the lower. I tried my friends bushmaster upper on my lower and it had the exact same issue, but my upper on his lower worked. (the problem seems to be isolated to my lower).

Just for kicks, I visually compared my and another (working) lower, and the dimensions and placement all seemed identical.

I'm super confused, any input will be much appreciated.

Thanks!

iskra31
05-31-2011, 12:05 AM
What kind of buffer are you using? I'm still fairly new to AR's. But it sounds like maybe a buffer and buffer spring problem.

AmbientR
05-31-2011, 12:25 AM
im using the ATI commercial/civilian buffer tube kit.. I think I got it from midway. I feel like it could be too shallow, not allowing the bolt to go all the way back, but the tube is the same length as on the other lower I tried... I'm not sure what brand that other one was.

Merc1138
05-31-2011, 12:32 AM
im using the ATI commercial/civilian buffer tube kit.. I think I got it from midway. I feel like it could be too shallow, not allowing the bolt to go all the way back, but the tube is the same length as on the other lower I tried... I'm not sure what brand that other one was.

Take a picture of your rifle, then take a picture of the buffer and buffer spring, that way we can see what the problem might actually be.

crazychinaman
05-31-2011, 6:43 AM
Are you using a rifle recoil spring in a carbine stock.? also buffer .

Barabas
05-31-2011, 6:54 AM
Does the bolt catch have spring tension on it? Does it move at all?

AmbientR
05-31-2011, 7:46 AM
Take a picture of your rifle, then take a picture of the buffer and buffer spring, that way we can see what the problem might actually be.

Ok, sorry for not doing that already. I will post some pictures tonight, thanks.


Are you using a rifle recoil spring in a carbine stock.? also buffer .

This sounds like it could be a possibility. I suppose I wasn't aware there was a difference. The buffer tube looks the exact same as a working rifle I compared it to, dimensionally, but I did not compare the springs and actual buffer... I will check that next.


Does the bolt catch have spring tension on it? Does it move at all?

The bolt catch itself appears to be functioning properly. It has proper tension and moves exactly the way it does on a rifle I compared it to.




Thanks for the input everyone. I will post pictures and new findings tonight as I get home from work.

AmbientR
05-31-2011, 11:10 PM
I hope these pictures help. Also, how does the buffer and spring look to u guys (too long, or what)? I was wondering about the rubber plug that seems to be at the end of the buffer.. it looks like it could be the same length as how short the bolt comes when fully pulling the charging handle.

http://i15.photobucket.com/albums/a374/Ambient_R/1-1.jpg
http://i15.photobucket.com/albums/a374/Ambient_R/2-1.jpg
http://i15.photobucket.com/albums/a374/Ambient_R/3-1.jpg
http://i15.photobucket.com/albums/a374/Ambient_R/4-1.jpg

AmbientR
05-31-2011, 11:18 PM
http://i15.photobucket.com/albums/a374/Ambient_R/5-1.jpg
http://i15.photobucket.com/albums/a374/Ambient_R/6-1.jpg
http://i15.photobucket.com/albums/a374/Ambient_R/7-1.jpg
http://i15.photobucket.com/albums/a374/Ambient_R/8-1.jpg

FeuerFrei
06-01-2011, 8:29 AM
Looks like the buffer is being hammered by the carrier.
The carrier should not mark up the face of the buffer that much.
You shot this rifle with out doing a function check? Carrier won't/didn't ever lock back?
My best WAG is buffer tube. Switch it out with a "known good" buffer assembly and recheck.

Hozr
06-01-2011, 8:31 AM
Spring doesn't look all that long, however, if the wire itself is too thick the spring will have a compressed length that is too long. Clear as mud? I had to cut a couple coils off my pistol spring because when it was fully compressed it was too long to allow the bolt to catch.

What are the compressed/uncompressed lengths of the spring.

AR-15 Rep
06-01-2011, 9:05 AM
There is definately a problem with the buffer being hammered. You may want to check your upper to the known good lower and check to see that the bolt carrier does not have a gap between the BCG and the buffer, reverse that with a known good upper to your lower. There should not be any gap between them and the Carrier should push the buffer back a little bit.

crazychinaman
06-01-2011, 9:23 AM
The buffer looks to long and the spring does too.

SpeedTribe
06-01-2011, 10:27 AM
Sounds like you may be running a rifle length spring. Measure it uncompressed to see what type you have.

From:
TECHNICAL MANUAL
ARMY NO. 9-1005-319-23&P
AIR FORCE TO 11W3-5-5-42

RIFLE: 11 3/4 Inches (29 85 cm)
minimum to 13 1./2 inches (34 29 cm)
maximum

CARBINE: 10 1/16 inches (25.56 cm)
minimum to 11 1/4 inches (28.58 cm)
maximum.

wash
06-01-2011, 10:41 AM
I have never had a problem like that but I've never bought an ATI buffer tube...

Your buffer looks like it has taken a beating, all of mine have only minor surface wear on their front face. Something is wrong there.

I wonder, how much do you save buying an ATI commercial stock kit?

My personal philosophy is to spend the $10 or $20 to a good get mil spec tube and a few more to get a Magpul CTR or another quality stock instead of one that you will regret later.

killshot44
06-01-2011, 11:31 AM
That IS a carbine spring and buffer.

The buffer looks beat up because of the delrin "cap", not an issue.

Everyone is overlooking I tried my friends bushmaster upper on my lower and it had the exact same issue, but my upper on his lower worked. (the problem seems to be isolated to my lower)

I'd guess the problem is the bolt-catch itself or possibly just a need to break it in. Since this potential fix is free why not lube the snot out of the bolt carrier and bolt, sit down and pull the bolt back till it catches and release it. Repeat 200 times.

SpeedTribe
06-01-2011, 12:13 PM
killshot44 is right. The coil count on the spring appears to be correct for an average carbine spring.

I overlooked the OPs buffer tube is ATI. You may want to measure the inside of the tube from the inside rear to the end of the tube. The inside length should be about 7". There were some posts online about people getting ATI buffer tubes with less than 6.5".
Here is a link (http://www.ar15.com/archive/topic.html?b=3&f=66&t=509477)..

rdmax
06-01-2011, 2:25 PM
Is there a delrin cap on the buffer? If so, remove the cap and try it. It is probably taking up too much space. I have a lower where the bolt did not pull back all the way also. I trimmed the rubber bumper on the buffer by 1/16". The carrier bolt locks back every time now. Maybe my buffer tube is out of spec. This was a cheap fix versus buying another buffer tube.

dieselpower
06-01-2011, 2:50 PM
Is there a delrin cap on the buffer? If so, remove the cap and try it. It is probably taking up too much space. I have a lower where the bolt did not pull back all the way also. I trimmed the rubber bumper on the buffer by 1/16". The carrier bolt locks back every time now. Maybe my buffer tube is out of spec. This was a cheap fix versus buying another buffer tube.

get rid of the commercial tube, get rid of the delron buffer, get rid of the crazy spring. The delron cap is causing the buffer to be too long, so when the spring is fully collapsed its not far back enough to let the BCG lock back.

I bet if you swap parts the BCG will manually lock back. Do that first. If you swap parts and the BCG still can not manually lock back, your bolt catch is pinned too low to allow full upward movement and get infront of the BCG.

wash
06-01-2011, 5:57 PM
A commercial tube isn't always a bad thing, it just can limit your choices when you are picking a stock.

What sounds like a really bad idea is buying a commercial buffer tube from a company known for cheesy products that may or may not work well (Mosin Nagant bolt handle).

A buffer tube might not be the sexiest place to spend your money in an AR build but as you can see here, it is kind of critical.

I think you might be able to save maybe $30 compared to buying a known good product.

I know some people have to pinch pennies but spending maybe $100 more than absolute bottom of the barrel parts in a few key places will give you a much better rifle without any surprises.

greybeard
06-01-2011, 6:24 PM
Not that it helps but I have the same thing with a 308 build. The buffer is for a 223 rifle. I am waiting on the correct shorter buffer to come.

killshot44
06-01-2011, 8:00 PM
killshot44 is right. The coil count on the spring appears to be correct for an average carbine spring.

I overlooked the OPs buffer tube is ATI. You may want to measure the inside of the tube from the inside rear to the end of the tube. The inside length should be about 7". There were some posts online about people getting ATI buffer tubes with less than 6.5".
Here is a link (http://www.ar15.com/archive/topic.html?b=3&f=66&t=509477)..

Is there a delrin cap on the buffer? If so, remove the cap and try it. It is probably taking up too much space. I have a lower where the bolt did not pull back all the way also. I trimmed the rubber bumper on the buffer by 1/16". The carrier bolt locks back every time now. Maybe my buffer tube is out of spec. This was a cheap fix versus buying another buffer tube.

get rid of the commercial tube, get rid of the delron buffer, get rid of the crazy spring. The delron cap is causing the buffer to be too long, so when the spring is fully collapsed its not far back enough to let the BCG lock back.


I overlooked the fact that the delrin part is probably making the buffer too long. And yeah, measure the internal depth of the tube, should be 7".

wash
06-01-2011, 8:15 PM
Is that a delrin cap or is the buffer a solid piece of delrin?

Neither of those are a good idea if you just want it to work.

Set it up in a configuration that should work before you try to "fix" it. As I've said, all of my buffers have minimal wear so there is no reason to add a plastic gimmick to the buffer unless you are trying to solve a problem.

If you take too many chances, you won't know where to look when something goes wrong.

xbimmers
06-01-2011, 8:22 PM
That buffer sure looks like the 9mm buffer I have in my 9mm AR. ......

I am betting that you are running a 9mm buffer. Switch it out for a light buffer or regular buffer and you will be fine.

Here is some info on running a 9mm buffer on a 5.56.

http://www.calguns.net/calgunforum/showthread.php?t=89945

AmbientR
06-01-2011, 9:07 PM
Thanks for all the responses everyone. All make valid points and all super helpful. This is my first AR so its nice to work out these problems because I'm really learning a lot about the rifle.

I noticed the wear on the buffer from the bcg as well, and thought it was normal.. (although i must admit it looks MUCH worse in the photographs). The back of the bcg completely contacts the face of the buffer (no space in between) when the rifle is assembled.

After doing a bit more reading, it looks like it might be a thing with the length of the ATI tubes. I guess I didnt realize they were so bottom of the barrel....

I'm going to measure the inside of the tube now. I'll edit with results.


That buffer sure looks like the 9mm buffer I have in my 9mm AR. ......

I am betting that you are running a 9mm buffer. Switch it out for a light buffer or regular buffer and you will be fine.

Here is some info on running a 9mm buffer on a 5.56.

http://www.calguns.net/calgunforum/showthread.php?t=89945

This is the one I got:
http://www.midwayusa.com/viewProduct/?productNumber=657408

EDIT:
Also, I just measured the inner length of the tube... Its 6.75''..... *sigh*... I could use that extra .25

Any suggestions on a replacement? I would only need the actual tube, right? not a spring and buffer? (my guess is just a tube).

Thanks again for your responses.

AmbientR
06-01-2011, 10:40 PM
Ok, I think maybe it's easier to cut the rubber bumper off by .25 inch or slightly less.

Thoughts?

http://i15.photobucket.com/albums/a374/Ambient_R/buffer1.jpg
http://i15.photobucket.com/albums/a374/Ambient_R/buffer2.jpg

AmbientR
06-01-2011, 11:30 PM
so I cut down the bumper at the end by a small amount, and it seems to just clear the bolt now. This basically means that it works the way it should now. Now if my mags were only strong enough to lift the bolt catch on their own :( (I'm guessing it might work when actual rounds are being stripped, I'll have to try that later.

http://i15.photobucket.com/albums/a374/Ambient_R/buffercut.jpg
http://i15.photobucket.com/albums/a374/Ambient_R/buffercut2.jpg

Before (With bolt fully retracted as far as it will go, notice its not far back enough for the bolt catch)
http://i15.photobucket.com/albums/a374/Ambient_R/before.jpg

After cutting buffer piece (The bolt is fully held back by the bolt stop)
http://i15.photobucket.com/albums/a374/Ambient_R/after.jpg

Can anyone tell me how much past the bolt catch the frontmost part of the bolt should be when it is fully retracted? This will help give me an idea if I need to grind down the buffer a little more or not..


Also, for the sake of earlier's discussion, here's another (better?) photo of the buffer face.
http://i15.photobucket.com/albums/a374/Ambient_R/bufferfacewear.jpg

stitchnicklas
06-01-2011, 11:31 PM
i recommennd that you go to ammo vros and spend a few bucks on a new buffer tube,spring and buffer,do a function check and if it works normally then ...BAM..problem solved and call midway and return the defective? ati thing

wash
06-02-2011, 8:54 AM
In an auto loading rifle, things move pretty fast.

Leaving just enough room for the bolt hold open to pop up in front of the bolt might not give things enough time for the bolt hold open to work. You are still left with a question.

The safe bet is to buy a commercial buffer tube and a new buffer (because after cutting, yours might be too short), the spring is probably OK.

If you want to upgrade the stock right now, I would suggest buying a mil-spec tube because when companies design new stocks, they always make the mil-spec version first and some times they never get around to making a comercial version. The only problem is that your commercial stock won't fit the new tube so you are forced to upgrade if you change types.

If your ATI buffer tube came as a kit with the stock and you are going to return the whole thing, going mil-spec might make sense.

As for choosing which parts to buy, the safe bet is to buy from a place that makes a good complete rifle. Spikes, Daniel Defense, BCM, RRA, there are lots of options. If a good place sells a generic buffer tube it is probably good to go. If you want to spend a little more you can get a brand name like Vltor (who make fine buffer tubes) or PWS (who make an enhanced buffer tube with an anti-tilt feature and QD sling attach points built in). If you want to go all out, the Vltor A5 system looks like it will deliver rifle buffer reliability with a collapsible stock.

The biggest lesson here is if you think you have found a way to save $30 on a rifle, google it to see if people have problems. If the results are inconclusive, ask somebody.

AmbientR
06-04-2011, 10:50 PM
I want to thank everyone for their input on this thread. I think I got the problem solved, and did not need to buy any new parts (yet)?

It looks like the issue was caused by a combination of the buffer length (which was fixed by cutting it, above) and also the parts just being all new and tight. After adding some rem oil to the pivot and mating points of the bolt catch, it started to loosen up a bit more. This allowed the mag follower to push it up all the way with ease, and my bolt now stops back properly when an empty mag is installed. It took a lot of wiggling to get the parts to break-in a little bit.

I guess I was afraid to over-oil in the beginning because of all the horror stories I have read about particles being stuck in/around moving parts on an AR. Guess thats just one of those learning steps.

:oops: