9mm AR - help me diagnose an issue

BigWaylon

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I built this one out last year, I think?

CMMG Colt-mag lower. PSA build kit. PSA 4” upper. Form 1 silencer. I think the buffer is 7oz...definitely a heavier/longer PCC style.

I’ve had it out a handful of times, and once every mag or two (or three) I have the same issue, or at least a very similar issue. I’ve just never taken the time to try and figure it out. Well today, it happened again while I was shooting it and while @Zbizzle911 was shooting it.

Basic description is I usually end up with an empty case sticking out the ejection port, with a new round being chambered by the bolt. Looks like this:

719669DD-BE34-463F-82D6-9C1C1BC4003E.jpeg

In this case, it was the last two rounds in the mag. But I’ve had it happen with the 2nd and 3rd rounds, two rounds out of the middle, and anywhere else. It’s happened with 20rd mags and 32rd mags...so I don’t think it’s mag related, especially since it’s stripping off the next round each time.

While it normally has a stove piped piece of brass, every once in awhile it’s this instead:

8D5B6EA6-A4B3-4FE1-8514-81F930DB9C8C.jpeg

...where the empty didn’t make it completely out but the fresh round was headed to the chamber.

It’s almost like the empty isn’t being tossed out like it should.

Any ideas on things to check? Things to test? I can’t intentionally make it happen. It’s never more often than once every 15-25 rounds...and sometimes it’ll go an entire mag or two trouble-free.

My 9mm SBR has never had this issue, so I’m not even sure where to start.
 
I had what appears to be the same issue with my direct blow back 9mm. For me is was the ejection port size of the upper reciever, I replaced it with an Aero XL upper and have had zero problem since.

I'm also using an Aero XL on my CMMG RDBB and couldn't be happier with the function of both of my 9 PCCs.
 
What’s the ejection pattern like? How does it compare to your SBR?


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I had what appears to be the same issue with my direct blow back 9mm. For me is was the ejection port size of the upper reciever, I replaced it with an Aero XL upper and have had zero problem since.

I'm also using an Aero XL on my CMMG RDBB and couldn't be happier with the function of both of my 9 PCCs.
The pistol is a complete upper from PSA. No ejection door cover and brass deflector is much smaller than a standard one.

The SBR has a complete upper from JSE that used a YHM 9mm upper. It has an ejector port cover and a standard looking brass deflector.

Maybe I can measure the opening size for both.

So you’re using one like this? https://www.brownells.com/rifle-par...5-assembled-xl-upper-receiver-prod126736.aspx
 
Check the extractor, spring, and o ring to make sure it has proper tension when extracting out the casing. I've had brand new rifles with out of spec extractors and springs cause very similar issues. If you have the same results, it could be the buffer weight and/or buffer spring. Try switching out buffers, then springs, then both, with the SBR, and see if you get the same results.
 
What’s the ejection pattern like? How does it compare to your SBR?


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When I was watching @BigWaylon shoot it today the ejection pattern was pretty erratic. I had my phone trying to video it in slow mo but of course it wouldn't do it while camera was on. I honestly thought that it appeared the spent casing wasn't ejecting and was just riding the bolt back forward while it was stripping a good round.
 
Check the extractor, spring, and o ring to make sure it has proper tension when extracting out the casing. I've had brand new rifles with out of spec extractors and springs cause very similar issues. If you have the same results, it could be the buffer weight and/or buffer spring. Try switching out buffers, then springs, then both, with the SBR, and see if you get the same results.
Thanks. Will probably just start by swapping BCGs and see how it goes. I’ll weigh the buffers in both as well.

Going from memory, I wanna say the SBR has a 7oz and the pistol has an 8oz.
 
Keep us updated on what you find out.
 
FWIW...

ejection port on the pistol is ever so slightly larger than the one on the SBR...same length (~3”), but I’m getting ~.01” wider
 
Check the ejector. When they get too loose they will have this problem, and also an erratic case ejection.
Never messed with one. So I’m not sure I’ll know what “too loose” is. :(

Not that there isn’t an issue, but this one was brand new and I’d say has <500 rounds, maybe less than 300.

Do they make different ones for 9mm? I know the bolt is for 9mm, so maybe I just buy a new ejector and see. see below
 
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Back up...ejector, not extractor, right?
 
This is a CMMG Colt-mag lower, and the ejector is already installed when you receive it.

F12FB005-24D7-4873-8145-0EA5045835D9.jpeg F8B3EC3C-BF75-4D51-A38E-AF82C124A00F.jpeg

I can’t notice any movement.

Wonder if I should slightly bend it towards the center just a hair?
 
This is a CMMG Colt-mag lower, and the ejector is already installed when you receive it.

View attachment 227782 View attachment 227783

I can’t notice any movement.

Wonder if I should slightly bend it towards the center just a hair?
I like mine tuned so that it just barely touched the channel in the bolt. They don't have much contact with the case so any little bit helps.
 
I like mine tuned so that it just barely touched the channel in the bolt. They don't have much contact with the case so any little bit helps.
Is there a way to see that? Just trying to picture it with the upper attached to the lower.
 
Looking up thru the magwell on mine can see it pretty clear. Also witness marks in the carbon that coats everything after a few rounds.
I’ll get it back out of the safe tomorrow and grab a flashlight.
 
Look at the end of the ejector and see if your case strikes are burnishing it “around the corner” of the front. If so it needs to be bent slightly towards the center.
 
Ok. These two pics should show it’s definitely bent a little more “out” than “in”. Wasn’t any fun trying to hold my phone to take a pic, flashlight in my mouth, and pull the CH back just enough...especially with the smaller colt-size magazine well.

64B970EA-F07F-4495-BA6F-3B77F2261B79.jpeg

D5FF8066-77FA-4D78-BAF8-DB455642E2CB.jpeg

Is the solution possibly as simple as bending the ejector so that it almost makes contact with the center instead of the outer edge?
 
Wasn’t any fun trying to hold my phone to take a pic, flashlight in my mouth, and pull the CH back just enough.
The solution is to sanitize that flashlight with alcohol. Somebody else might need’ta use it. :eek:
 
Ok. These two pics should show it’s definitely bent a little more “out” than “in”. Wasn’t any fun trying to hold my phone to take a pic, flashlight in my mouth, and pull the CH back just enough...especially with the smaller colt-size magazine well.

View attachment 227961

View attachment 227963

Is the solution possibly as simple as bending the ejector so that it almost makes contact with the center instead of the outer edge?

yes
 
15938901305075061566890322923348.jpg mine's not even close to centered and works 100%. Tosses brass to 3:30/4:00.
 
View attachment 227989 mine's not even close to centered and works 100%. Tosses brass to 3:30/4:00.
Any chance you can get a pic up through the magwell so I’ll see where it sits relative to the BCG?

im going to go look at my SBR and see how it lines up.
 
It's not really visible, it rides in the channel . You can see a sliver of bronze metal in this one. That's it.
 

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SBR is definitely closer to the center side of the channel. The point might be in about the same place of the one I bent, but it’s more like the whole thing is leaning over instead of bending the end like I did.

923A2801-F424-4FE8-A32B-04A1488120EF.jpeg

I’m not going to try to bend the end back and bend the whole thing instead since the ejector is integral to the lower. I guess it might be replaceable, but not sure.

I’ll hold off on trying anything else until I shoot it again. But I will take the SBR with me and swap the BCG if I have additional failures.
 
Do you have dummy rounds to cycle by hand?
 
Do you have dummy rounds to cycle by hand?
Yes. But I’ve also ejected multiple live rounds out of it and never had an issue.

It’s always been the every once in awhile issue during live fire. It’s nothing I’ve been able to make happen when I want it to.
 
Gun looks dry IMO.....

Lube the crap out of it and see if it runs..

Hope you get it sorted out!
 
D3BF8E42-510F-44B4-9655-5B163FB556A4.jpeg

Snap caps barely fall out...but it launches a live round a decent distance.
 
Here's my theory. The ejector is 1/16" too short or the buffer is 1/16" too long.
The bolt travels far enough to pick up new rounds, but not far enough to eject them consistently. When you get a slightly underpowered round the bolt travels with less force to the rear and doesn't bottom out travel with sufficient force to make positive contact with the ejector. It's like a limp wrist on a pistol causing a stovepipe.
 
Here's my theory. The ejector is 1/16" too short or the buffer is 1/16" too long.
The bolt travels far enough to pick up new rounds, but not far enough to eject them consistently. When you get a slightly underpowered round the bolt travels with less force to the rear and doesn't bottom out travel with sufficient force to make positive contact with the ejector. It's like a limp wrist on a pistol causing a stovepipe.
I did have one quarter in the tube and removed it at the range. We still had one (maybe 2?) of the same issue after I removed it.

Ammo is primarily 147gr. Every once in awhile I’ll run some 115 or 124 to show somebody the difference in sound with and without the sonic crack. Just can’t say either way whether I’ve had the issue with anything other than 147 or not.
 
Just measured buffers. The one in the firearm is ~3.9” and the other I have here is right at 4”. The one installed almost looks smushed on the bumper.

I pulled the one from the SBR and it’s just under 3.7”. It’s a Hahn, and they have them in stock. It’s about an ounce less than the one currently installed as well. Although it’s 3x the price of the one I’m running, I might go ahead and order one.
 
According to Foxtrot Mike people experience issues with buffers longer than 3.8". By issues, I think they're referring to burst fire because dwell time is too short. Just the same, probably best to avoid the accidental machine gun and run a buffer that doesn't enable that in the first place.

I'm running a 6.5oz FM 9mm buffer that is 3.8". I'm also using a standard AR buffer spring, but they recommend a .308 spring. I don't run suppressed (yet).
 
Standard carbine spring in the SBR. That’s what I was running in the pistol, but swapped to a Damage Industries enhanced spring (still carbine size) before the outing yesterday. Same issue before and after.

I went ahead and ordered the Hahn buffer since it’s worked well in the SBR. I’ll swap it before taking it back out. At that point, I don’t care if it’s the buffer or ejector as long as it works. Plus, I won’t be wasting ammo testing the ejector mod if it’s really the buffer. :D
 
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