My friend just built a 9mm SBR, all CMMG

CMMG dedicated 9mm Lower
Slash 9mm-Q buffer with XP Spring
Standard CMMG single stage trigger

CMMG Upper
8.2" CMMG Barrel
CMMG 9mm BCG

We've run into a problem, and I'm hoping someone can help.  I did a search, which gave us some troubleshooting, but now I need some more specific help.

The CMMG doesn't seem to want to work with 115gr bullets.  124gr, 147gr, 157gr, all work fine.  The 115gr however have FTE every other round.  Doesn't matter on brand.  We've swapped buffers, swapped buffer springs, added quarters, tried every trick we know, and still getting a consistent FTE.  Even checked the extractor on his BCG by putting it into another gun, fired/cycled fine.  I think it's a buffer problem.  We've now got a bin of spare 9mm AR parts to swap and mix and match, so wanted to reach out to the group and see if anyone else is having this problem.  If so, do you have a solution?  Goal is to get it to fire 115gr reliably.  I've thought through the physics of how the blowback system is working, and my thoughts are as follow, someone please correct me if my thinking is wrong:

115gr bullets are leaving the barrel too quickly to generate enough pressure to force back the BCG - solution I would have thought would be lower buffer weight.  Tried, but problem remains
BCG is unlocking too quickly with not enough force to completely cycle/extract - solution I would have thought would be heavier buffer weight and spring.  Tried, but problem remains

Magazines are 9mm 32rd mags from Brownells.  Function just fine in my 9mm 5.5" AR, and feed just fine in his with anything NOT 115gr.

Any thoughts?  What haven't I thought of?  Thanks in advance for the help

So, by FTE, do you mean "failure to eject", vs "failure to extract"?

Also, your BCG does not "unlock". It's direct blowback, and is always unlocked. If you think it's moving back too soon, you should increase the weight in the BC, I should think.

I don't have that issue with my QC10 build that uses a CMMG 8.5" barrel. But something I have noticed (and may contact CMMG about) is that I am getting what appears to be excessive blow by on the cartridges? Not sure if the barrel (chamber) is out of spec or what but I see evidence of a lot of blow by that I don't see with any of my other firearms.

How do your spent cases look?

I'm looking at failure to eject.  Stovepipes.

I meant to say that I thought its moving back too quickly, poor use of words, my bad.  However, how can I change the weight of the BC?  I know that there is a weighted insert in the CMMG carrier, but I didn't think I could change it

I've looked at the casings, and they don't seem to be any additional wear or unusual deformations.  I've only been using factory ammo during this, no reloads.  No additional blowback that I can tell

Thanks for the input everyone. I'm going to run another test with him on Friday.  Do some more side by side comparisons.  My QC10 build hasn't had any of the same issues, so I'm thinking of running his upper on my lower, since I'm running a different buffer system.  Also going to switch BCG, just incase, even though we're both running CMMG, maybe something is out of spec?  It could be just as simple as his build is more ammo sensitive, which I hope isn't the case

The rounds that do eject, what's the position and distance? If they land in the same position at varying distance then adjusting the ejector may do no good. still, have a look at the relation between the ejector groove in the bolt and ejector, it should be high and tight on that TINY spot it has to hit on the case yet not dragging on the carrier. I'm thinking the extra bolt speed firing 115's is causing ejection problems that a Minor ejector tweak will fix.

I'll take a look at the ejection pattern on Friday.  Will post up range results when I get back.  THanks again for the assist

Buy a heavier one., it's the most cost effective way to go.

Any brand suggestions?  Most manufacturers don't list the weight.  CMMG, RRA or JP.  Any other 9mm BCG makers worth looking at?

I honestly would look at a few more thing before you start throwing more money at it. The setup you have should be working.

If you cycle it manually (slowly) can you feel any "sticking" at all? Check and make sure the ejector isn't rubbing on the bolt. I had to ever so slightly file a little bit off the back of mine when I first got it. Just something else to look at?

Good luck

Try a different buffer.

Thanks everyone for your input.  Here is the update:

We took it to the range again, and it once again jammed up on 115gr.  So we took a known, fully functional weapon 9mm AR SBR, and began to swap parts one at a time.  My friend's CMMG upper worked just fine on my QC10 lower with UCIW 9mm Heavy Buffer, 10 for 10 in the test.  We then mounted my 5.5" Vltor upper with his CMMG lower with Slash 9mm buffer, and it worked just fine.  However, once we mate the CMMG upper with the CMMG lower, all hell breaks loose.  We compared BCG, even swapped them, still failure to eject in his.  Absolutely puzzling.  So he's reaching out to CMMG today, because something must be out of spec that we can't see.  And since this started life as a CMMG pistol, it should be covered under warranty.  I'll let everyone know how it goes

Just out of curiosity, when you mated the CMMG upper and lower (with bolt in etc) did you notice if it was difficult to get the rear pin in? Did you have squeeze the upper and lower together some to get it in?

No issue mating the CMMG upper to the lower.  There WAS some difficulty mating my MUR 9mm upper to his CMMG lower though.  Not enough to be a problem, but it was noticeably more difficult on the rear pin

Strange? I'm out of ideas. My MUR upper on my QC10 lower was extremely tight as well. The ejector was hitting the bolt so I took a little off the ejector to get it to fit properly. Thought that may have been an issue withe the CMMG upper but doesn't sound like it. Good luck!