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Discussion Starter #1
My .223 build works great except it doesn't always lock open on an empty mag. The latch moves freely. It locks when you pull the charging handle on an empty mag. I've tried it on several mags, doesn't seem to be a specific one. They will all lock open cycled manually.

It doesn't seem to be ammo-related. Though my reloads aren't "hot" (maybe 5% below max), it also does it with commercial ammo.

At first, I thought it was a "dry" recoil spring that I failed to lube at assembly. I lubed it, and it seemed to lock open sometimes. It seems to move freely in the tube. I've weighed buffers, and they're within spec. I traded buffers and spring with my other carbine, and it seemed to help some, but at the range yesterday it failed to lock open again.

I was wondering if a gas tube was mis-aligned, and it was under-gassed, but the empty cases eject smartly at 1:30, about 6-8' away from the bench. So I don't think so.

It's an Anderson lower. I've built guns on 2 other Andersons, and they haven't given me problems.

suggestions?
 

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Not an expert by any means, but a video that I watched on building mine said that ejection should be at 4 to 5 oclock if properly gassed. Less than 4 indicates under and more than 5 indicates over gassed. However, as with most things, there any many varying opinions on this. My Son in laws had a slight leak around the gas valve and gave us a few problems but we solved that fairly easily. What length gas system and gas tubes are you using? Are the other rifles the same systems or are different?
 

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The bolt is not going back far enough or the magazine spring is not strong enough. Those are the only two real possibilities.
Switch magazines first because it's easiest to test and discount.
 

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Elk Whisperer (Super Moderator)
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My .223 build works great except it doesn't always lock open on an empty mag. The latch moves freely. It locks when you pull the charging handle on an empty mag. I've tried it on several mags, doesn't seem to be a specific one. They will all lock open cycled manually.

It doesn't seem to be ammo-related. Though my reloads aren't "hot" (maybe 5% below max), it also does it with commercial ammo.

At first, I thought it was a "dry" recoil spring that I failed to lube at assembly. I lubed it, and it seemed to lock open sometimes. It seems to move freely in the tube. I've weighed buffers, and they're within spec. I traded buffers and spring with my other carbine, and it seemed to help some, but at the range yesterday it failed to lock open again.
The bolt is not going back far enough or the magazine spring is not strong enough. Those are the only two real possibilities.
Switch magazines first because it's easiest to test and discount.
Sounds like the OP did that already :confused:

RJ
 

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That is a sensible enough fix to try two magazines before messing with gas ports. ;)
IN my opinion, of course.
I'm too lazy to tackle every problem the hard way.
 

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What buffer are you using? If the it is locking open when manually pulling bolt to rear, the it isn't the magazine nor the bolt hold open device. Underpowered ammo or too heavy of a buffer.


CD
 

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Discussion Starter #7 (Edited)
I've had this problem on and off a couple years, and I'm pretty sure it's not the upper, b/c when I put it on my other carbine lower, I got no malfunctions. But I'm going to repeat that experiment.

Ejection is at 1 to 3 o'clock which most say is enough or over-gassed (3 -4 is ideal, 5 or more is under-gassed). I don't think you'd get strong ejection with a too-heavy buffer or spring. So it pretty much has to be the bolt catch.

I have 7 or 8 Colt 20 round magazines, they're all probably 30+ years old; they may be a little tired, but they work in my other 2 guns.

The bolt catch operates freely, it catches the magazine follower on all mags, and catches the bolt every time it's manually cycled. One guy on AR-15.com said sometimes debris can be in the spring recess for the bolt catch, or it isn't drilled deeply enough, or the spring can be too long, any of which will give you too much spring tension on the bolt catch. It seems its only job is to keep the bolt catch down until the magazine follower pushes it up. You'd think if it can push the bolt catch up when you manually cycle it, it would when fired, unless the bolt isn't coming back far enough, or the catch comes up too slowly.
 

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Discussion Starter #8 (Edited)
I'm learning everything I can to solve my problem. Here's what I've discovered so far. Hope it helps someone.

I had Patrick Sweeney's book on the AR (vol. 1) on the shelf and I've re-read it. He doesn't get into this specific malfunction a lot, but his understanding of the system is very helpful.

So there are 2 possible causes: 1. the bolt not cycling back far enough, 2. the bolt catch not rising (so far - DUH)

the bolt not going back far enough may be caused by:
a. the system being under-gassed due to low-powered ammo, leak or blockage in the gas system - gas port too small or blocked, gas tube leak or poor fit, loose carrier key (leak), worn gas rings. The design of the AR is such that with proper parts, assembly and ammo, the possibility of a low-mileage AR (at least a carbine) being under-gassed is very small.
b. restricted / delayed bolt movement - anything that creates drag on the system: rough or dirty chamber, inadequate lubrication of bolt, carrier, or recoil spring, improper or too heavy a buffer or spring, dents or restrictions in the buffer tube, rough hammer face (?)
If the bolt is cycling back far enough, but is not caught by the bolt catch, it has to be either:
a. a magazine issue - weak springs or follower tilting - or
b. something with the bolt catch itself - either it's not the right one, it's rough or broken, or something in the cutout is restricting its movement: dirt, grit, filings, or too strong a return spring to be overcome by the magazine spring.
It's rare, but I've heard of oversized mag wells that let the magazine wobble, causing the follower not to engage the bolt catch.

I'm betting magazines are the culprit a lot of the time.

What else am I overlooking? now, I gotta apply all this to mine, and eliminate 1 thing at a time to find what's causing it.
 

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Discussion Starter #9
to Blackhawk's question, I have 2 16" carbines, both with carbine-length gas systems and collapsible stocks - one in 5.56 (the problem child) and one in 300 BLK. I also have a 20" in .223 with a solid stock. The 20" .223 and the Blackout both run all my magazines fine.
 

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We were told to set ours up with carbine-length gas systems too. We went with 16" barrels as well. It is very possible that you could have a slight gas leak, or the gas system may have a slight block in it. It sounds like to me that for whatever reason, the bolt is not going far enough back. May not be the best way, but after installing the gas port and tubes on Ours, we plugged the gas tube on the bolt end of barrel and blew into the barrel. On doing this, we discovered that my Son-in-laws had a slight gas leak around the port. We took it off and realigned it, tested the same way and it worked fine. This seems to be a really easy way to check for a leak.
 

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Discussion Starter #11 (Edited)
I read a trick recently, you can put light thumb pressure on the bottom of the bolt catch while firing the last round. If the bolt comes back far enough, it will definitely lock. I'll try it my next trip to the range. I also learned it's possible to accidentally swap the bolt catch and disconnector springs while building. They're the same length, but slightly different. Seems hard to imagine that would be enough to make a difference, but I'm gonna eliminate every variable I can find.
 

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I've had this problem on and off a couple years, and I'm pretty sure it's not the upper, b/c when I put it on my other carbine lower, I got no malfunctions. But I'm going to repeat that experiment.

Ejection is at 1 to 3 o'clock which most say is enough or over-gassed (3 -4 is ideal, 5 or more is under-gassed). I don't think you'd get strong ejection with a too-heavy buffer or spring. So it pretty much has to be the bolt catch.

I have 7 or 8 Colt 20 round magazines, they're all probably 30+ years old; they may be a little tired, but they work in my other 2 guns.

The bolt catch operates freely, it catches the magazine follower on all mags, and catches the bolt every time it's manually cycled. One guy on AR-15.com said sometimes debris can be in the spring recess for the bolt catch, or it isn't drilled deeply enough, or the spring can be too long, any of which will give you too much spring tension on the bolt catch. It seems its only job is to keep the bolt catch down until the magazine follower pushes it up. You'd think if it can push the bolt catch up when you manually cycle it, it would when fired, unless the bolt isn't coming back far enough, or the catch comes up too slowly.
I had these issues with my AR10.
I got a shorter spring and it works fine now. There is a flat coil spring also available. I did not go that route.
2 inch shorter spring. Compare the length of your buffer tube with your other rifles. Mine is shorter. So standard spring simply takes up too much space. AR 10s are not standardized like AR15s.
 

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Discussion Starter #13
I'd compared buffer weights and spring lengths; hadn't thought about measuring buffer tube lengths. I'd just assumed they were standard. I sort of doubt that's the problem, because the bolt can cycle manually far enough to engage the bolt catch. But it's worth checking.
 

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Discussion Starter #14 (Edited)
It's the upper. Here's why I'm pretty sure: I took 2 carbine lowers and 6 (numbered) mags to the range yesterday, and it failed to lock open on all 6 mags n both lowers. However, ejection was brisk, with the empties falling 6-8' at the 1:30-2"30 position, AND light thumb pressure on the bottom of the bolt stop latch locked it open EVERY time with all mags on both lowers, so the bolt IS coming back far enough. So, I either have 6 weak magazines (but they all work every time in my 300 BLK and my 20" AR) or it's over-gassed.

You wouldn't think over-gassing would cause the malfunction, but as I understand it, the carrier/buffer should be approaching zero velocity as it gets to the back of the buffer tube, lightly bump the back of the tube, and then spring forward. I think mine is fast enough it's bouncing off the back of the buffer tube, not giving the latch the split-second it needs to come up and catch the bolt. I probably need a slightly stiffer spring and/or heavier buffer (?). I don't want to get into an adjustable gas block. Does all that sound right?
 

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Sounds like the cheapest way to go would be to get a heavier buffer and stiffer spring. Have to check prices and see which is best. I am thinking of changing mine to an adjustable buffer, and if I'm not mistaken have seen where mid south shooters has one on sale now for 39.99 plus shipping. Not sure what buffers and springs cost. Also, I watched a video that suggested that one may want to have at least a few different weight buffers on hand anyway............................................ Good luck let us know when you solve this problem.
 

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The next thing to try is replace the bolt lock itself. The size and shape of the surface that contacts the magazine follower determines how well it works.
 

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Discussion Starter #17
H1 buffer is ordered. Several (apparently good) videos and other articles pretty much agree a lot of carbines are over-gassed, and mine appears to be. Too fast a cycle can cause premature wear on bolt lugs, harsher than normal recoil, and some feed issues. Slowing down the bolt/carrier slightly seems like the solution. We'll see....
 

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Discussion Starter #18
Solved. But it was a combination of 2 things - slightly dirty mags and buffer.

The buffer came (quicker than expected - thanks Primary Arms and USPO.) so I went to the range. First I tried to duplicate the malfunction with 3 mags, 1st mag locked open once, not 2nd time. The next 2 failed.

On a hunch, I opened #1 and # 2 to compare spring lengths and followers. The springs looked the same, but I noticed some carbon and maybe a little crud on the back of the follower, so I cleaned the mags. I embarrassed to admit I'd never done that - I just assumed because they worked in my other guns, and I seldom/never dropped them in the dirt, they were ok. I wiped them out, paying a lot of attention to the rectangular slot the back of follower rides in. Not filthy, but the paper towel got a good bit of carbon on it. I wiped the follower very lightly with oil, reassembled and tested. They locked open about 50%.

Changed to H1 buffer, wiped what I could reach at the top of the other 4 mags without disassembling, ran 1 round through all six, twice. All locked open every time. And the H1 buffer moved the ejection from about 1:30 to 2:30, and feels maybe a little smoother-running.

They're all getting torn down tonight at home, cleaned thoroughly. Leave the follower dry? lightly oil? Renaissance wax?

I feel a little dumb, but I hope this helps somebody else.
 

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The Shadow (Super Mod)
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I'm not big on lubricating mags, but we have very fine volcanic soils here that get everywhere; think of flour.

If you do, remember that a lot less is more. As has been said on the forum here:
Enough oil that if added to fine scotch, wouldn't ruin the taste.

Cheers
 

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Glad you got it sorted out. Good information. Will help someone else a lot. I watched several videos before and during our builds, and I don't recall anyone addressing overgassing the system. It would be nice to have some sort of trouble shooting manual....................... Sort of like "Chiltons" for autos.................................
 
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