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Gunsmithing Help: AR15 Bolt not locking back on last round

Rootshot

Rootshot

Sergeant
Full Member
Minuteman
Mar 28, 2006
314
149
South Florida
#1
I just built a "frankenrifle" AR15 Carbine. It shoots well and is perfectly reliable so far, but the bolt consistently fails to lock open on the last round.

It locks open just fine if cocked manually with an empty magazine. It has the same behavior over 5 different magazines.

Here are the specs:
- Armalite mil-spec upper, bolt carrier, and bolt.
- Nordic Components 1-in-8 stainless mid-length gas system 16" barrel with compensator and PRI low profile gas block.
- LAR Grizzly lower with mil-spec buffer tube and M4 stock. I bought it used and think the buffer and spring are standard. It's built using a DPMS lower parts kit and an unknown 3lb match trigger.
- Magazines are 10rd DPMS polymer magazines. (flawless in other rifles)
- Ammo if Lake City 08 brass, Wolf 223 primers, 25.5gr Varget, 75gr Hornaday match.

My guess is that it may be under-gassed, that the recoil spring is too strong or that the buffer is too heavy. I'm not sure how to narrow it down.

Thanks for the help.

--Rootshot
 
#2
Re: Help: AR15 Bolt not locking back on last round

Are you 100% sure the gas block is properly lined up? Also does it feel like your bolt stop has any drag when you push it by hand or is it nice and smooth?
 
#5
Re: Help: AR15 Bolt not locking back on last round

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Outsy</div><div class="ubbcode-body">Load the mag with 1 round, chamber it, shoot it, and see if it locks back then. If not, I would look at the buffer first and see if it is an H buffer.</div></div>

if it were the buffer, wouldn't it not cycle period?
 
#6
Re: Help: AR15 Bolt not locking back on last round

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lazy21</div><div class="ubbcode-body"> <div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Outsy</div><div class="ubbcode-body">Load the mag with 1 round, chamber it, shoot it, and see if it locks back then. If not, I would look at the buffer first and see if it is an H buffer.</div></div>

if it were the buffer, wouldn't it not cycle period? </div></div>

The BCG may be cycling enough to come back over the round and strip it out of the mag, but not far enough back to catch on the bolt catch.

He says he can lock the bolt back manually, so the bolt isnt too long (which I have seen).

It also could be spring pressure in the magazine not pushing up the bolt catch.

But I would look at the buffer first.
 
#7
Re: Help: AR15 Bolt not locking back on last round

with the M4 stock, you should have a carbine buffer and spring.

check mag, try different one. if same issue then look at gas block for alignment.

if sure its 100% double check gas rings, make sure they aren't lined up(if new this shouldnt be much of an issue)

Still a problem, send it to someone who works on these rifles.
 
#8
Re: Help: AR15 Bolt not locking back on last round

Thank you for the help. I checked the gas block alignment and it was canted by a few degrees. Unfortunately the barrel did not have flats machined at the bottom for the gas block set screws, so alignment is a bit of eyeball guesswork.

I corrected the gas block alignment and will see if this solves the problem next weekend.

Thanks for the input.

--Rootshot
 
#10
Re: Help: AR15 Bolt not locking back on last round

The problem still persists...

- Checked the buffer. It's a standard weight carbine buffer.
- The spring appears correct, but I don't have another carbine spring to compare it to.
- The gas block is aligned properly.
- There is enough gas to properly cycle the rifle. It I put finger pressure on the bottom of the bolt release ping-pong paddle, the bolt will lock open. This is an indicator that the bolt carrier assembly is moving far enough back.
- I've tried other magazines and the problem persists.
- I've replaced the bolt catch assembly with a new one without effect.

At this time, I'm almost out of ideas. My only other thought is that the rifle is over-gassed and that the bolt moves back into battery fast enough to strip a round, but too fast to lock in the open position. My next experiment will be to try a heavier buffer to slow it down.

Any other ideas?

Thanks,

--Rootshot
 
#11
Re: Help: AR15 Bolt not locking back on last round

After trying a heavy buffer, replacing the bolt release, using lower power rounds, and a few other things that didn't work, I gave up and replaced the lower receiver with a new one and installed a new lower parts kit. The only parts that were moved to the new lower were the trigger, stock, buffer tube, buffer, and buffer spring.

Still no luck. Same problem. Any thoughts?

--Rootshot
 
#13
Re: Help: AR15 Bolt not locking back on last round

Is there any paint on the end of the buffer spring. Since you bought it used there may be a chance you have a heavy duty spring in there. I tried a couple from "Tactical Springs" and didn't have much luck with the rifle functioning correctly. One was the heavy duty painted red and one was normal painted blue. I went with the standard buffer spring and from a H to 2H back to the standard buffer. Also measure the spring length. IIRC 10 1/16 - 11 1/4" is the acceptable range. A shorter length is a sign of a worn spring. Too long and it requires more force to get it to cycle properly (there are those that try and pull them apart to regain the length).
 
#14
Re: Help: AR15 Bolt not locking back on last round

I had the same problem with my AR-15 and it turned out the spring for the bolt release was too heavy. Too heavy for the magazine follower to overcome the force. I went to a lighter weight spring and all was good.
 
#15
Re: Help: AR15 Bolt not locking back on last round

Is your gas tube going into the gas key deep enough to seal off well enough to get a full cycle? Is the gas key tight and the bolt staked into place? If your spring and buffer are the correct weights, I suspect you are losing gas pressure somewhere. Did you nick the gas tube during assembly possibly, or possibly the nut that retains the barrel that the gas tube runs through may have come loose and popped a hole in it?
 
#16
Re: Help: AR15 Bolt not locking back on last round

Sacshooter,

I may have to try that as a last resort, but I don't think that's it. I took the original lower (which was causing me trouble) and put a rifle length buffer assembly and A2 stock on it as well as my short 16" Grendel upper. Everything works great and locks back just fine. Prior to this, both the Grendel upper and 5.56 upper failed to lock open on the same lower. Since there were two different uppers exhibiting the same problem, it has to be in the lower.

Since I replaced all parts in the lower including the lower itself (except for the buffer spring, buffer tube and trigger), this has me thinking it has to do with the buffer tube or spring (trigger shouldn't matter and has worked fine in a different lower). Since I already replaced the buffer with a Spike's powdered tungsten heavy buffer, and it is unlikely that there is a defect in the buffer tube, I have to believe that the buffer spring is worn out and that it cycles too fast. Next step is to replace the buffer spring with a new one.

FYI - Removing and replacing or installing the roll-pin for the bolt catch is such a PITA. One trick I found was that I can use a red bendy-straw from a WD40 or other aerosol can as a slave pin from the front to hold everything together while I hammer in the roll pin from the rear. Works great!
 
#17
Re: Help: AR15 Bolt not locking back on last round

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: SA_Friday</div><div class="ubbcode-body">Is your gas tube going into the gas key deep enough to seal off well enough to get a full cycle? Is the gas key tight and the bolt staked into place? If your spring and buffer are the correct weights, I suspect you are losing gas pressure somewhere. Did you nick the gas tube during assembly possibly, or possibly the nut that retains the barrel that the gas tube runs through may have come loose and popped a hole in it? </div></div>

Everything looks just fine. The key is staked and tight. There are no signs of gas leakage. If I press the bottom of the bolt release paddle while firing the bolt will lock. This means that there is adequate pressure.

Thanks for the help.

--Rootshot
 
#18
Re: Help: AR15 Bolt not locking back on last round

Rootshot
I recently had this exact problem and went through some of the same things you are going through to find the problem. Replaced buffer spring, multiple different magazines etc.

the thread is here:
http://www.snipershide.com/forum/ubbthre...410#Post2254410

but the culprit in the end was an improperly lubed BCG. I was using tetra gun grease, and when it was hot, it did just fine. When it started getting cold here, it seriously gummed up the works. Stripped everything with carb cleaner, lubed with CLP, and every one of my ARs that was having problems locking open were cured.

Just a thought, and easier to test than replacing a bunch of parts.

madd0c
 
#19
Re: Help: AR15 Bolt not locking back on last round

What happens if you pull the charging handle back with a empty mag and let go of the charging handle?

I have seen a few that the bolt stop does not come up far enough to engage the bolt properly. The issue is in the lower itself, not the bolt stop. Either the dowel pin hole is off or the pocket that the bolt stop pivots into is shallow. If it is this, it is easily fixed be shaving some off of the bolt stop down below the pin hole where it goes into the lower. This will allow your bolt stop to come up further and give a more positive contact with your bolt.
 
#20
Re: Help: AR15 Bolt not locking back on last round

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: heatseekins</div><div class="ubbcode-body">What happens if you pull the charging handle back with a empty mag and let go of the charging handle?

I have seen a few that the bolt stop does not come up far enough to engage the bolt properly. The issue is in the lower itself, not the bolt stop. Either the dowel pin hole is off or the pocket that the bolt stop pivots into is shallow. If it is this, it is easily fixed be shaving some off of the bolt stop down below the pin hole where it goes into the lower. This will allow your bolt stop to come up further and give a more positive contact with your bolt. </div></div>

It locks open just fine if I manually operate the charging handle on an empty mag. Since the failure is persistent across 2 lowers (LAR Grizzly and Aero Precision) it should be isolated to the components I moved from one lower to the other (buffer spring, buffer tube, trigger assembly).

Both uppers lock open properly on my Seekins Precision lower (which has a rifle length buffer system).

My guess at this point in time is that the carbine length buffer spring is worn out and that the buffer is hitting the back of the buffer tube and bouncing back too fast. Slow enough for the magazine to feed a round, but not fast enough to engage the bolt catch on an empty mag.

Thanks,

--Rootshot

p.s. Glen - your lowers are flawless both cosmetically and functionally
smile.gif
 
#21
Re: Help: AR15 Bolt not locking back on last round

you have a midlength gas system,it not going to be over gased. if anything it maybe under gased.this assuming its been properly drilled. with that set up,I see no need for a heavy buffer. with a small rubber hose you can blow through the gas tube,to makes sure it,passes air easily. do you have other carbine springs on hand?
 
#22
Re: Help: AR15 Bolt not locking back on last round

As hard as it is to guess at these things on a forum, it sounds like the problem is isolated to the spring and buffer if the lower works w/ rifle buffer and spring. Do you want me to send you a carbine buffer and unused spring to test?

Also, since the problem goes away w/ the rifle buffer, tube, and spring, is this a 6 pos. stock? If so have you shot it fully extended to see if it's a coil bind thing?
 
#23
Re: Help: AR15 Bolt not locking back on last round

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: fngmike</div><div class="ubbcode-body">As hard as it is to guess at these things on a forum, it sounds like the problem is isolated to the spring and buffer if the lower works w/ rifle buffer and spring. Do you want me to send you a carbine buffer and unused spring to test?

Also, since the problem goes away w/ the rifle buffer, tube, and spring, is this a 6 pos. stock? If so have you shot it fully extended to see if it's a coil bind thing? </div></div>

I really appreciate the offer for you to send me a buffer spring. I can't think of another online community where strangers help each other like this! I have a spring on order and still have the standard weight carbine buffer, but really appreciate the offer.

I always shoot it fully extended (most comfortable for me). Not sure if it is 5 or 6 position. It's a milspec tube with a (older style?) carbine stock. Not sure what "coil bind" is.

Thanks,

--Rootshot
 
#24
Re: Help: AR15 Bolt not locking back on last round

Your welcome for the offer, not sure I'll ever use the things, LOL. Sorry for the coil bind reference, it's an old gearhead term. If you install new valve springs on a cylinder head they have a specific installed height and if the height is not correct when they compress to open the valve the spring coils can bind on each other.

If your spring is too long and you were shooting from one of the collapsed positions with your stock I was wondering if that was occurring. If that was happening though you'd probably notice more recoil.

Hope the new spring solves it, let us know either way.
 
#25
Re: Help: AR15 Bolt not locking back on last round

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: fngmike</div><div class="ubbcode-body">Your welcome for the offer, not sure I'll ever use the things, LOL. Sorry for the coil bind reference, it's an old gearhead term. If you install new valve springs on a cylinder head they have a specific installed height and if the height is not correct when they compress to open the valve the spring coils can bind on each other.

If your spring is too long and you were shooting from one of the collapsed positions with your stock I was wondering if that was occurring. If that was happening though you'd probably notice more recoil.

Hope the new spring solves it, let us know either way. </div></div>

Just cause the stock moves, doesnt mean the length of the tube changes .....
 
#27
Re: Help: AR15 Bolt not locking back on last round

I would try heavier lubrication and a new action spring.

I bet that clears everything up.

Most people look at gas systems first, and I have no idea why. Of the dozens of AR's I troubleshot with folks on this very issue, 1 had a gas problem, and that was with an aftermarket gas block.

Insufficient lube will add too much friction to the system, and stall the carrier out sooner (getting it far enough back to strip a round, but not lock back). After bad magazines, under-lubrication seems to be the biggest problem I see with AR's.
 
#28
Re: Help: AR15 Bolt not locking back on last round

Check your bolt stop overrun.

Put an empty mag in the gun.

Pull the charging handle all the way to the rear and hold it.

Check how much gap there is between the Catch and the Bolt face, it should be about .075 inches (2mm). This is about the thickness of a nickel. If it is any closer than this, it is your problem.

If it is too close, you can sand or file some off the plastic snubber on the rear of the Buffer.

I'd say your buffer or buffer tube is out of spec. A coil binding spring (too long) can also cause this.
 
#29
Re: Help: AR15 Bolt not locking back on last round

My guesses would be the mag spring (are mags used or new?), the gas system, the buffer spring, then the bolt stop.

Who built the bolt carrier and staked the key? Try dribbling a little oil along the sides of the key. If there is ANY looseness, it will show here by sucking in oil and squeezing it out. Just because it looks solid does not mean that it was staked unsupported and bent the cap screws. Also, try the oil trick and blow through the key, see if there is any movement of oil around the key.
Can you feel the gas tube slide into the key? Can you see the bulge on the tube, and is it a bit shiny from use?
What gas block are you using? Is it held in with two roll pins or some kind of set screw? You mentioned "Franken-AR", so just curious.
How new are the rings? Did you overlap any of them?

Are mags new or used? Try Pmags if you can get your hands on one.

Buffer spring: Is this new or used? Mil-spec or some aftermarket spring? Swap it out for a new Mil-spec carbine length spring with H (maybe H2) buffer.

Is the bolt stop and spring an aftermarket, high zoot kind of thing? Or is it a mil-spec? Is it out of a GI lower parts kit?

I have found a bunch of problems when you start mixing and matching aftermarket components in a "Franken-AR".

Just a few ideas is all.
 
#30
Re: Help: AR15 Bolt not locking back on last round

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ogreshooter</div><div class="ubbcode-body">My guesses would be the mag spring (are mags used or new?), the gas system, the buffer spring, then the bolt stop.

Who built the bolt carrier and staked the key? Try dribbling a little oil along the sides of the key. If there is ANY looseness, it will show here by sucking in oil and squeezing it out. Just because it looks solid does not mean that it was staked unsupported and bent the cap screws. Also, try the oil trick and blow through the key, see if there is any movement of oil around the key.
Can you feel the gas tube slide into the key? Can you see the bulge on the tube, and is it a bit shiny from use?
</div></div>

Factory Armalite bolt carrier and bolt assembly upgraded with a Wolff extractor spring. Staking is correct and there is no movement. There is no evident tube wear.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ogreshooter</div><div class="ubbcode-body">
What gas block are you using? Is it held in with two roll pins or some kind of set screw? You mentioned "Franken-AR", so just curious.</div></div>

It's a PRI low profile gas block with two set screws at the bottom. I actually took it off and checked alignment. Carbon markings indicate it is properly aligned. Was put back on in the same position with blue locktite on the set screws.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ogreshooter</div><div class="ubbcode-body">How new are the rings? Did you overlap any of them?</div></div>

The rings have about 1,500 rounds on them and are factory original Armalite. I have checked for overlap and they do properly overlap.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ogreshooter</div><div class="ubbcode-body">Are mags new or used? Try Pmags if you can get your hands on one.</div></div>

I'm using 10rd DPMS polymer mags. Pmags (unless modified to reduce capacity) are not legal in California. The same failure occurs with a Grendel upper and C-Products magazines. I've borrowed a few other magazines at the range. Pretty sure it's not the mags.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ogreshooter</div><div class="ubbcode-body">Buffer spring: Is this new or used? Mil-spec or some aftermarket spring? Swap it out for a new Mil-spec carbine length spring with H (maybe H2) buffer.</div></div>

The buffer spring came with the used lower receiver. I have no idea of the round count and this sounds like the suspect part that is failing. A replacement has been ordered. I have already replaced the standard carbine buffer with a heavier powdered Tungsten buffer from Spike's Tactical without change in behavior.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ogreshooter</div><div class="ubbcode-body">Is the bolt stop and spring an aftermarket, high zoot kind of thing? Or is it a mil-spec? Is it out of a GI lower parts kit?</div></div>

In the latest test, I am using a brand new DPMS lower parts kit in a brand new Aero Precision lower.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ogreshooter</div><div class="ubbcode-body">I have found a bunch of problems when you start mixing and matching aftermarket components in a "Franken-AR".</div></div>

Tell me about it!!!
laugh.gif
Since I actually enjoy learning about the mechanics and nuances of the guns I down, this has been a masochistically fun game.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ogreshooter</div><div class="ubbcode-body">Just a few ideas is all. </div></div>

Thanks,

--Rootshot
 
#31
Re: Help: AR15 Bolt not locking back on last round

Root,

I had my latest carbine doing the same thing, ended up being the bolt catch (undersized Mil-Spec part). It was very loose fitting in the lower, rocked side to side considerably. I measured it up, and found one with a little more width to fit my lower better. I installed it, problem solved. Maybe that will help.

Z
 
#32
Re: Help: AR15 Bolt not locking back on last round

Well, my replacement buffer spring came in the mail right before the weekend. I purchased a David Tubb Flat Wire Chrome Silicon spring (might as well try something new). This spring is considerably heavier than the stock carbine spring.

Did it work? Yes and maybe. Reliability / functioning is perfect. While engaging the bolt catch with my finger while shooting, the carrier consistently locks back on every shot. This is a good indication there is still enough gas to overcome the enhanced spring and heavy buffer.

The bolt does lock open on the last round about 60% of the time. This is better and confirms that it was related to the buffer spring, but it still doesn't solve the problem. I gave the rifle a good cleaning and oiling and will try again next week with some new magazines.

--Rootshot
 
#33
Re: Help: AR15 Bolt not locking back on last round

I joined the group so that I could jump into this briar patch. A neighbor asked me to fix his M4 by CMMG, which was not locking after the last round in the magazine. In addition to my internet search, which brought me here, I perused my books for any other insights. First, I straightened the front sight post and enlarged the gas port to about .090", which previously has cured short-stroking in AR-15s with shortened barrels (16-1/4"). I also bought a carbine length action spring, which was 10" long. The original spring was 9-1/4".

The problem persisted, and based on the experience of Barrett Tillman in his 2000 book about the M-16, I examined the cases closely: sure enough, the rim of the case was slightly bent by the extractor. His idea (and of some others, too) is that the carbines are "overgassed" and that the bolt tries to extract the case before chamber pressure drops and the expanded case shrinks to permit easy extraction.

So, after some contemplation and trepidation, I performed the fix (#1) offered by BT: I threaded the rear end of the gas tube 6-40x and inserted a 1/4" screw which I faced and drilled through .070". I also applied some red thread locker designed to hold pistol compensators. This operation reduced the gas vent to about 40 percent of the factory gas tube ID of .113". I also installed an action spring provided by the owner which measured 11" long. It then worked well with ammo by both PMC and Win-USA, didn't eject the cases so far, the base remained flat, and it just didn't sound so violent during firing.
 
#34
Re: Help: AR15 Bolt not locking back on last round

I have had this problem and have found it in at least one of my buddies rifles. Once in a while they will make a buffer that has a roll pin that is either too long or has walked a bit.

Take a really really close look at the roll pin on both sides of your buffer and if it projects at all, file/grind/smooth it down.

Even enough to feel but not really see can cause it to catch on your spring and prevent full travel.
 
#35
Re: Help: AR15 Bolt not locking back on last round

The malfunction saga is finally over! Even after all the other things I tried, there were still problems with the bolt not locking back.

I ended up replacing EVERY part in the lower receiver (including the lower itself) except the buffer tube, end plate, and castle nut. I finally replaced these parts as well and that did the trick. 7 magazines with zero malfunctions.

Although I can't see anything visibly wrong with the original 4 position milspec carbine buffer tube, it appears to be out of spec in some manner.

Thanks again for everyone's suggestions.

--Rootshot
 
#36
Are you using factory ammo or reloaded ammo? and what caliber are you shooting? 223 Rem, 5.56 NATO, 300 Blackout, or 308 Win.?
As long as the rifle chambers another round from magazine, it would be consider normal function.
As for the bolt carrier to lock back on last fired round, it's more of a safety feature, allowing shooter to see that the magazine is empty.
But for your question, correcting it would allow you to say the rifle is working correctly and is safe.
Do the process of elimination, you want your rifle to function at 100% or better.
The process may cost a few extra pennies, better to be safe than sore.
Remember! SAFETY FIRST ! fun after!
 
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