AR15 bolt won't lock back after last round

[LiquidTension]

I've read all of the various threads on different boards I can find and still haven't been able to fix the problem. First, the gun:

CMMG 11.5" complete upper
Spike's lower
DPMS parts kit
Tube, spring, and stock from my M&P15
Daniel Defense heavy buffer
Yankee hill lightweight rail
Magpul sights
Magpul BAD lever
Magpul MIAD grip
AAC 18T Blackout

The bolt will lock back manually.

Possible problems:
1) Undergassed. The original FSB had some leakage around the gas tube. Thinking that I may have messed it up somehow when I was turning it into a homemade low profile block, I replaced both the gas block and the gas tube with new parts. It now wears a YHM low profile gas block and a new tube. This had no effect on the problem. While I had the gas block off I measure the gas port. It appeared to be a few thousandths too large, so it should be getting enough gas.

2) Incorrect spring in the mag catch. I was quite careful when assembling the lower, and the catch rides up to where it is supposed to be on the bolt, meaning that I did not accidentally put the disconnector spring in there. The channel for the catch is free of debris and has been lubed. No effect.

3) BAD lever. This part has been known on some rifles to cause the bolt not to lock back on an empty mag. Lever was removed. No effect.

4) Buffer system. I swapped the heavy buffer with a standard buffer. No effect. The tube is free of debris, spring is fine, tube is lubed.

6) Bad magazine. The problem persists with multiple examples each of HK mags, Pmags, and USGI mags.

7) Ammo. I usually run PMC and have not had any problems. While troubleshooting I also ran some 75gr loads and a number of other random loads. No effect.

The lower put on a different upper runs perfectly. The upper on a different lower will still not lock the bolt back consistently. This seems to point to a problem with the upper. Swapping bolts out increases the chances of the bolt locking back, but only up to about 70%. Also worth noting, the CMMG upper is very difficult to mount to all of the lowers I have tried, which is admittedly few since I always have to procure someone else's registered lower in order to assemble it.

The upper may be slightly out of spec, but I don't know which dimensions to check that could be responsible for this issue. Over a dozen calls to CMMG have gone unanswered, and leaving them a message to call me back is not an option since I'm at work and calling between assignments.

Any ideas?

 

[Sky]

?? Cmmg may be on an extended holiday for Turkey Day?!

I have two CMMGs uppers (actually complete rifles) that I swap with Bushmasters and Del-Tons but all mine fit and work as advertised. It sounds like to me if the fit is not proper that you got a warped lower. Just a total dart in the dark on my part only because all my CMMGs fit everything I have and work? The 11.5 gas system seems to give problems unless properly tuned. But you said when you swap your lower it works but the fit is not all that? May have two problems going on or maybe I read you wrong?

I have only called CMMG a couple of times but they have always answered my calls.

 

[rcmodel]

The lower put on a different upper runs perfectly. The upper on a different lower will still not lock the bolt back consistently.

Well, theres your answer.

The problem lies in the overly short 11.5" barrel.
They get very picky when they are that short.

You will either have too much gas and beat the crap out of the action with 5.56 NATO, or not enough to work right all the time with all .223 ammo.

I imagine CMMG defers to 5.56 NATO pressure and there-in lies the problem.

Your 100% Gayronteed fix would be a longer barrel with the gas port where it is supposed to be.

And I would call them again after the holiday as Sky mentioned.

rc

 

[Sky]

rcmodel probably hit the nail on the head.

 

[One-Time]

Its a gas issue w/ the short upper, try som actual 5.56 Im betting the .223 is too weak or the upper isnt gassed enough to get it locked

 

[taliv]

I disagree with remodel on this one. There are far too many running sbr out there to say one won't work just because it is an sbr.

Did you try swapping just the bolt carrier?

 

[taliv]

The bolt catch spring would not affect this. It pulls the catch down not pushes it up. It is the mag spring/follower that push it up to stop the bolt on the last round

Cmmgs response is disappointing.


Do your bolt rings hold the carrier up when you stand it on end extended?

 

[LiquidTension]

It's something in the upper. I swapped the bolt and carrier out and put them in my M&P. The M&P has NEVER had any type of malfunction, but when I put the bolt from the CMMG in it, I had the same issue just not to the same extent. This leads me to believe that it's a combination of the bolt/carrier and the upper.

Taliv - yeah, I'm aware of the function of the mag catch spring. I'd read on (I think) M4c.net that when assembling the lower, people have accidentally swapped similar springs from the LPK and it has been known to cause this problem. I checked it just to rule out that possibility. As far as the rings holding up the carrier, I'll check that here shortly. I also agree with your assessment about there being so many SBRs running perfectly - for some reason I couldn't post a response in this thread for the rest of the day from my office computer.

 

[taliv]

did you put the M&P carrier in the CMMG upper/lower and see if it will run?

 

[One-Time]

weak ammo or too small of a gas port or too heavy of a buffer

 

[tommytman]

Check you gas tube alignment. Take the bolt out of the carrier and the upper off the lower. Now insert the carrier alone in the upper untill you can see it through the eject port with upper parallel to the floor. Now gently tilt the upper muzzle down. The carrier should slide forward once you get to 30-45 degrees of tilt and it should slide all the way forward uder it's own weight. If it hangs up on the gas tube that is a problem. Post back.
ps. also try this: fire one round while pushing down the bottom of the bolt catch gently. This will force the catch to engage if the bolt travels back far enough. note where the catch engages, does it engage on the bolt face every time ? it can also engage on the bolt carrier and you don't want that.

pps: also pull the carry handle back (with rifle assembled) and see where the bolt face position is. Should be about 1/4 inch in front of the rear of the eject port and behind the bolt catch.

also: is the bolt carrier riding the mag lips??? also when you pull the charhing handle is the resistance uniform throughout the whole recoil stroke? or is it stiffer on the last couple of inches of travel?

 

[Al Thompson]

Just to throw some more mud on the windshield, a number of random rounds (as LT mentioned) were 5.56.

I'm puzzled by this too.

 

[clem]

Make sure that the carrier key screws are tight and staked.

 

[amlevin]

When I had this problem with my 11.5" bbl AR, I ended up solving the issue by replacing the gas rings. I was using a McFarlane one-piece and after a few rounds it started this problem. New 3-piece rings and I haven't had a problem since. Assuming the gas tube is properly aligned in the gas block, the bolt carrier is clean, and your ammo is not underpowered the gas rings are a good place to go looking. Check for proper "gap spacing" too.

 

[LiquidTension]

The CMMG bolt in the M&P upper gave me the only malfunction I've ever had out of the M&P, which happened to be the same problem with the bolt not locking back. However, it only did this with one magazine (20rd Pmag). It worked fine with the HK mag, a 30rd Pmag, and a USGI mag. As Al stated, we used multiple random types of ammo in multiple bullet weights with the same results.

I haven't been able to mess with the thing for the past few weeks due to the holidays, but I'm hoping to take it out this weekend. The gas rings were spaced evenly apart, but I still need to check if they'll support the carrier. The gas key is staked properly.

 

[rcmodel]

The gas rings were spaced evenly apart

That should make no difference.
If it does, you have something else out of whack.

Those three gas rings are constantly moving on the bolt and align themselves off & on all the time.

A properly functioning AR should still be reliable with one ring missing completely.

rc

 

[LiquidTension]

Yeah but I figured someone was going to throw that one out there at some point.

 

[LiquidTension]

Finally got back to the range.

Taliv - yes, the rings will support the carrier when the bolt is extended.

I ran the gun pretty wet this time and had pretty much the same results. M&P bolt and carrier in the SBR - would lock back fairly consistently on all magazines except the HK. Put BAD lever on and got significantly more failures. Removed BAD lever again.

CMMG bolt and carrier in M&P - random failures to lock back, but not nearly as bad as when running the complete CMMG package.

I finally got through to CMMG and spoke to Jordan. He was fairly certain it was a problem with being undergassed even though the gas port is slightly larger than spec and the bolt and carrier cause the malfunction in a different gun. They're paying for me to send the upper back to them and get it tuned correctly, even after I replaced the gas block and tube. I have to say that I'm perfectly happy with that and now have no complaints about customer service. They could have easily blamed me for the problem since I swapped some parts out, but they're making it right anyway.

 

[Al Thompson]

Cool beans!

 

[LiquidTension]

People still say that?

The only thing I'm worried about is that if they just overbore the gas port it may cause the bolt to lock without actually addressing the real problem. Remember that the CMMG bolt and carrier cause malfunctions in other guns as well, so it can't be solely the fault of the gas system.

 

[nulook45]

It could also be an obstruction in the gas tube i got a kit with a m-4 barrel the thing was they manufacture drilled the gas port but left some small filing inside the gas tube drove me nuts:banghead: untill i striped the tube off and ran a 16 inch pipe cleaner thru it. i also had to clean gunk out of the bolt carrier and key same problem. rifle ran like a top when i was done.
CMMG is pretty good about fixing things good luck with it let us know how it turns out >