AR-15 not locking back on last round

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GDownRange

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So I built my first AR-15. I did it because I like to tinker with things. I followed a hodgepodge of videos from the usual sources e.g. Brownells, Midway. I really enjoyed the internet research, and actual assembly. Unfortunately my first attempt was not a success because the rifle is not locking back on the last round.

First trip to the range I fired 20 rounds of .223, one round in a mag, 20 times. The rifle never locked back. If I pulled the charging handle with an empty mag installed the bolt locked back every time. I brought the gun home cleaned it thoroughly. I didn't see any damaged parts, or any wear outside of the charging handle.

Second trip to the range I fired 46 rounds using a combination of .223 and .556, with 12 different mags, and the bolt never locked back. I started out with the adjustable gas block tightened all the way down. The bolt stayed locked and the gun functioned like a single shot. Pulling the charging handle back locked the bolt open as expected. I started opening the gas block about 1/2 a turn and firing a round. As soon as the bolt opened and ejected a round, the bolt did not lock back. I then kept opening the gas block in 1/2 turn increments and fireing a round until the gas block was completely open. The rifle ejected the round and returned to battery on every shot. For six shots I kept my head up to watch the ejection angle. All 6 rounds ejected around 3 o'clock.

Based on what the various internet sources say, it seems I have a gas issue. Can anyone who has built their own AR and had a similar problem give me advice on the steps I should take to trouble shoot?

I have a 16" barrel with a mid-length gas system and an Odin Works adjustable gas block. If any other specs or details are needed please let me know.
 
My first guess would be an alignment issue with the gas block. But also, what kind of buffer and tube are you using?
 
It’s a mil-spec tube with a carbine buffer.

If the buffer/tube were causing the problem, how would I troubleshoot?
 
If opening up the gas block all the way and it did not lock the bolt back I would check the alignment with the gas port as previously suggested. I'm assuming it locks back when you pull the charging handle back with a empty mag. The gas block does not go all the way to the barrel step normally there is a gap of ~0.025". If you slid it all the way one you may be partially blocking the port.

May want to look at the bolt piston rings (gas seals). Make sure the gap of the 3 rings are evenly spaced and not aligned up with each other. Use some oil on the rings when you install the bolt. The oil will help with the gas seal till it burns off. Check for a loose gas port on the BCG. I had a neighbor bring me his and said it quit locking back and had ejection problems. Who ever staked them did a poor job, did not hold.
 
I will take the gas block off and check the alignment. It is currently flush with the barrel step, so that may be my issue.

I have tried to wiggle the gas key, but there isn’t any noticeable give. If it were lose would I see it move with hand pressure?

The gas rings were evenly spaced both times I cleaned and assembled the BCG, but I didn’t oil the rings. I’ll try that as well.

Thank you for the advice.
 
What bolt catch are you using? If it is a lever one making it ambidextrous and the bolt locks when using the charging handle I start to suspect interference with the shooter.

Bolt catch spring and plunger installed correctly?

Only other thing I'm thinking is the gas port is not aligned properly making the regulator ineffective.
 
Insufficient gas (port too small, misalignment, leak at the gas block and/or bolt carrier key, bad gas rings), worn or out of spec magazine, mag catch and/or bolt catch, weak mag spring, worn follower.... These are a few of the things that I would be checking.
 
The follower is the part that engages the bolt catch. That's why I suggested trying some USGI or Colt magazines.
 
It's a machine, so malfunctions are systematic. Only a few things can cause failure to lock back:

1) Over length buffer bumper. Test: Can you pull the charging handle far enough to lock the bolt back by hand? If yes, then this is not it. If no, cut/sand the tail of the rubber bumper to give you sufficient clearance. The forward face of the bolt should draw approx 0.25" behind the bolt catch when fully rearward. If the bolt travel is gritty, exhibits any catches, or presents significant drag, there may be some other damage or dimensional interference between the BCG, buffer, and receiver extension - this should reveal itself in the buffer length test.

2) Insufficient travel of the bolt catch. Sticky, slow, or obstructed travel will prevent the bolt catch from catching the bolt. That can be poor fitment of the pin, catch, and receiver, obstructed/poorly lubricated plunger or spring, interference between the receiver (especially billets) and specialty catches, interference between the shooter and release extensions, etc. If it's not readily springing into place, it may not catch the bolt. Test: simply check the travel of the bolt catch to ensure it is responsive and without any hitches. Also confirm the catch is traveling sufficiently, not just catching at the tip, but fully blocking the bolt.

3) Insufficient bolt travel during cycle, typically due to improper gas system performance. This can be a loose gas key, poor gas ring seal, out of alignment gas block, plugged gas tube, undersized gas port, insufficient dwell time, under powered ammunition, and gas leaks in the system (block to barrel, block to tube, tube to key, key to carrier, and gas rings). Tests: Much of this can be checked by pulling the upper, removing the BCG, inserting an empty case into the chamber, and blowing into the muzzle. Look for carbon blast marks around the block on the barrel, on the gas tube where it inserts into the block, and on top of the carrier where the key is seated. Also inspect the end of the gas tube and the mouth of the gas key for carbon build up or torching. Confirm the gas rings are in good shape, and confirm the gas key is tight to the carrier (yes, if it's loose, you should be able to induce shift by hand).

4) Insufficient mag follower travel. This can be caused by dirty mags, weak springs, or damaged mags. Test: try a different mag. If a new mag works, clean the old one, check the spring and replace if necessary.

5) Damaged/worn bolt catch. Very rare, but it does happen. Chipped, broken, rounded, peened, or otherwise malformed bolt catches can let the bolt slip. Visually inspect the bolt catch, if it's not showing a crisp edge and flat face, replace it.
 
What Blue68F100 and Cz9Shooter said above are my best guess. The other would be that the gas tube was crimped or somehow damaged on installation. Varminterror gives a pretty thorough rundown of the other possibilities.
 
I agree with Varmintterror, specifically #5. Since this is your first built I'm thinking the spring for the bolt catch is not properly centered and is causing interference not allowing the catch to raise and catch the bolt on the last round.
 
So I built my first AR-15. I did it because I like to tinker with things. I followed a hodgepodge of videos from the usual sources e.g. Brownells, Midway. I really enjoyed the internet research, and actual assembly. Unfortunately my first attempt was not a success because the rifle is not locking back on the last round.

First trip to the range I fired 20 rounds of .223, one round in a mag, 20 times. The rifle never locked back. If I pulled the charging handle with an empty mag installed the bolt locked back every time. I brought the gun home cleaned it thoroughly. I didn't see any damaged parts, or any wear outside of the charging handle.

Second trip to the range I fired 46 rounds using a combination of .223 and .556, with 12 different mags, and the bolt never locked back. I started out with the adjustable gas block tightened all the way down. The bolt stayed locked and the gun functioned like a single shot. Pulling the charging handle back locked the bolt open as expected. I started opening the gas block about 1/2 a turn and firing a round. As soon as the bolt opened and ejected a round, the bolt did not lock back. I then kept opening the gas block in 1/2 turn increments and fireing a round until the gas block was completely open. The rifle ejected the round and returned to battery on every shot. For six shots I kept my head up to watch the ejection angle. All 6 rounds ejected around 3 o'clock.

Based on what the various internet sources say, it seems I have a gas issue. Can anyone who has built their own AR and had a similar problem give me advice on the steps I should take to trouble shoot?

I have a 16" barrel with a mid-length gas system and an Odin Works adjustable gas block. If any other specs or details are needed please let me know.
I would say you can rule out the bolt being too fast, given the fact that you only opened the gas block up by a half turn each round, so I would assume that its undergassed, as your bolt catch and magazines seem to be working fine. I'm assuming you've tried different mags and at least one was new right?

I also do not believe that the gas block being against the shoulder is a problem, as I've never seen a gas block that wasn't generous enough to handle that. If not using a handguard or if the barrel isn't dimpled under the gas port then the block really should be against the shoulder for support.

Do you have any way of measuring the gas port? If not, can you contact the manufacturer and ask them what it should be, then perhaps verify it with a drill bit of that size? In meantime, give it a good looking over. Also inspect the area around the gas block and the gas tube to see if there are any leaks, which will be obvious by streaks of carbon.

ETA: Also do this, with the lower removed from the upper, press down on the bolt catch, then pull it up. I've run across some lately that stick. Dangest thing.
 
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I just got caught up at work after the long holiday weekend and started to trouble shoot based on the feedback in this thread.

Fist thanks to everyone for all the guidance.

I pulled the hand guard off and can see carbon marks on the gas tube where it meets the block. Is a clear 1 inch carbon mark on the top of the gas tube a sign of a gas leak? If so how do I identify the out of spec part, (block, tube, barrel), or correct the problem?

I've also done the following and I believe everything checks out.
  1. Applied considerable pressure (with only my hands) to the gas key. There is no perceivable movement.
  2. The bolt does lock back when pulling the charging handle back with an empty mag.
  3. I don't have any USGI or Colt mags. If I can't figure the problem out I'll borrow or buy one
  4. I've fired 12 different mags, 6 PMAG 3rd Gen, and 6 MFT, all are new. (this is my first STANAG fed rifle)
  5. Removed the gas block and measured the shoulder to center hole distance. It matches the distance from barrel shoulder to gas port center almost exactly. I can also see a faint carbon circle almost perfectly centered around the gas port on the barrel.
  6. I can stick a 5/64 punch into the gas port on the barrel.
  7. I have a standard non-ambi bolt catch (included in a basic lower parts kit). It appears to move freely. I don't feel the bolt catch binding or grinding with the upper and lower separated, or when inserting/removing an empty mag.
  8. Gas rings appear to be in perfect condition, and were still somewhat spaced after firing 46 rounds.
  9. The gas tube looks fine (with the exception of the carbon mark noted above) and slides in and out of the barrel nut freely.
I think I've looked into all the great guidance given in this thread. The only thing I've found is evidence of gas leaking at the gas block towards the gas tube. Do we think this is the issue?
 
Is a clear 1 inch carbon mark on the top of the gas tube a sign of a gas leak?
That sounds like the culprit to me. Can you post a photo? What brand is the gas block?
Applied considerable pressure (with only my hands) to the gas key. There is no perceivable movement.
I believe your gas key is gtg.
I've fired 12 different mags, 6 PMAG 3rd Gen, and 6 MFT, all are new. (this is my first STANAG fed rifle)
That would satisfy me. I do not believe magazines are your issue. I also doubt that there's anything out of spec regarding the magwell or mag catch. To me, it sounds like the mag, magwell, and bolt catch are all where they should be and working just fine.
Removed the gas block and measured the shoulder to center hole distance. It matches the distance from barrel shoulder to gas port center almost exactly. I can also see a faint carbon circle almost perfectly centered around the gas port on the barrel.
That's normal. It's because the gas port in the gas block is intentially about twice the diameter of the gas port in the barrel, to make up for any slop. I've never seen a gas block so poorly aligned as to cause any issues. As long as the carbon is contained in that little circle and you don't see any streaking leaving that seal then you're fine.
I can stick a 5/64 punch into the gas port on the barrel.
Do you have calipers? If not, could you buy some at Harbor Freight? They come in handy, and they're really cheap. I don't think a punch is going to tell you anything, as they're not precise enough. A drill bit would be better.

I think the gas tube/gas block fit is the problem, but it wouldn't hurt to check the gas port as well. More often than not, these things tend to be multiple little problems stacking on top of each other. It might be that the gas port is a little undersized.

Also do me a favor and see if when the bolt is all the way back if you can fit a nickel between the bolt and the bolt catch.
 
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Below are the only 2 pictures I had enough forethought to take before I started cleaning everything. The gas block is one of the newer Odin Works adjustable set and forget.

I got out my pair of cheap-o calipers. It looks like the gas port is .078. I wouldn't bet my life on that measurement, but it's close. This is a 16" barrel with a mid-length gas system.

The carbon didn't show up well in the pick, but was very clear to the naked eye. As a note it did wipe clean with some solvent and a rag.
a>


t%20and%20b_zps4zyctmmt.jpg

Here is a pic of the port side of the tube. The carbon is caked on and did not wipe clean with a rag and solvent.
tube_zpsjt0ndwmt.jpg


Does this look like enough bleed off to under gas the rifle?
 
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Nobody has mentioned this yet, so here was my situation. On my rifle, I had exactly the same problem you are describing and tried everything you have been trying. I finally called the barrel company (Criterion) and asked them for advice. They said to drill the gas hole in the barrel a little larger. I did that and everything has been fine since. I was a little surprised to hear them advise that, but they said it is not uncommon to have that happen (which I also was surprised to hear).

Seeing the carbon deposits on your gas tube doesn't convince me you are getting excessive bleed off. The tube to block fit is not perfect and some bleed is probably expected.

P.S. See Blue68f100's post below that supports what I was trying to say.
 
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Nobody has mentioned this yet, so here was my situation. On my rifle, I had exactly the same problem you are describing and tried everything you have been trying. I finally called the barrel company (Criterion) and asked them for advice. They said to drill the gas hole in the barrel a little larger. I did that and everything has been fine since. I was a little surprised to hear them advise that, but they said it is not uncommon to have that happen (which I also was surprised to hear).

Seeing the carbon deposits on your gas tube doesn't convince me you are getting excessive bleed off. The tube to block fit is not perfect and some bleed is probably expected.

Thank you for the tip. I’ll shoot a quick email over to the barrel manufacturer, but according thier website the port is in spec (maybe even slightly oversized)

I’ll also try and slide a nickel between the Bolt and bolt catch when I get home tonight.
 
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Blank post entered in error. I’ll edit with an update tonight.
 
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Below are the only 2 pictures I had enough forethought to take before I started cleaning everything. The gas block is one of the newer Odin Works adjustable set and forget.

I got out my pair of cheap-o calipers. It looks like the gas port is .078. I wouldn't bet my life on that measurement, but it's close. This is a 16" barrel with a mid-length gas system.

The carbon didn't show up well in the pick, but was very clear to the naked eye. As a note it did wipe clean with some solvent and a rag.
a>


View attachment 769711

Here is a pic of the port side of the tube. The carbon is caked on and did not wipe clean with a rag and solvent.
View attachment 769712


Does this look like enough bleed off to under gas the rifle?
That is definitely a gas leak. Whether or not it's enough to cause your problem I can't say, but since it's the only thing apparently wrong I would start there. Did you buy the gas tube from Odin, as well?

On the gas port, I would leave it alone until you've ruled out everything else. It can always be opened up, but it doesn't work so well going in the other direction. If, however, you find that it's still not working after the gas block has been corrected then you can open it up a little. With 5.56, it really should be running just fine at .078.
 
Some gas leakage is expected, what you have is what most consider normal. If you have some true 5.56 ammo, your gun will probably operate since it runs at a higher pressure. But most all 556 ammo is tamed down some.

If your gas port is in fact 0.078" that is the min spec. This could be your problem. The spec range is 0.078-0.081" . The last barrel I made/turned and fitted, the min spec hole did not give enough gas to operate the bolt, just like your having. Drilling the hole out to mid spec corrected the problem. This is best done with number drills and/or reamers. I set mine up in my mill to drill it out. If you do this run a brass rod down the barrel to prevent from damaging it when you go through. Then you have to clean out the chips and any flashing still stuck to the barrel. A good fitting jag and patches works fine just takes several runs.
 
what you have is what most consider normal
I wouldn't consider it normal myself. The fact that he was able to remove the gas tube is also somewhat telling. Once fired, if in spec, it should be an ordeal and a half getting it back out.
 
The fact that he was able to remove the gas tube is also somewhat telling. Once fired, if in spec, it should be an ordeal and a half getting it back out.
Really? Why is that? I have never heard of that before. Maybe the carbon deposits makes it fit tighter?
 
Really? Why is that? I have never heard of that before. Maybe the carbon deposits makes it fit tighter?
They glue it in. And it only takes a few rounds to really cement one in there. To get it out without damaging it, you have to soak it in something for days.
 
They glue it in. And it only takes a few rounds to really cement one in there. To get it out without damaging it, you have to soak it in something for days.

Well that could be my problem. I didn’t glue the rod into the block. I simply pinned the rod into the block. Is using adhesive to attach the rod to the block a common/recommended practice?
 
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