So I've been debugging a 9mm upper on my RR for over a year now, off and on.  I seem to solve one problem, only to run into another problem.

Initially, I was running into issues with bullet seating depths and bullet profiles.  Figured out that my barrel had basically no throat.  I've had to rework all new seating depths for my reloads.

So the latest problem is that it runs perfect in semi, but in full, I get one round and then a dead trigger (hammer down).  Ejection is fine.  Next cartridge is chambered fine.  I can manually cycle it and start over.  Same result.  My experience with 5.56 uppers suggests that I have a bolt bounce issue.  I've read everything I can find on ideal buffer/carrier weights.  Everyone seems to suggest 22-24 oz. as ideal.  My bcg is at 16.5 oz and my extended buffer is just under 8 oz, so I'm already at 25 oz.  Does it sound like I should go even heavier?  I see there are 10 and 11 oz buffers out there.

Lower runs fine in semi and full with multiple 5.56 uppers, although I did switch to an H3 buffer as one upper had a similar issue as above.  Someone on here said it was bolt bounce and that was the solution. (and they were correct)

Thanks.

Colt upper.  4.5" barrel.  Not suppressed.

The buffer needs sliding weights,  make sure if it.
The firing pin needs it's spring, make.sure of it.

And check to make sure the firing pin isn't broken.

I have personally seen these problems on AR9 platforms, that had odd glitches.

Also, make sure the auto sear drops the hammer at the correct distance.
If too early or late, you can have timing issues.

If your 5.56 works well, and this one doesn't.  The bolt trip may be off. Try a buddy's bolt to see if there is an improvement .

It took me a while to sort mine out. Back then ramped bolts didn't exist  

I'm not a full auto guy, so take this all with a grain of salt, but I understand the basic principles from troubleshooting binary triggers.

I agree that it sounds like a timing issue or a bolt bounce issue.

You're getting a hammer down, no bang.

This could be because either the sear trip is releasing the hammer too early, or the bolt is bouncing out of battery backwards and catching the hammer as it swings forward.

To me that could mean the sear trip surface on the bolt is incorrect for this setup, and/or the bolt+buffer is moving at the wrong speed (too slow) for the sear trip timing, and/or you need a buffer with sliding internal weights

A lighter buffer or stronger spring might do the trick to bring the timing into sync if that's the problem.

A buffer with sliding internal weight should prevent bolt bounce.

An H3 5.5oz (don't forget to add a buffer spacer in the back of the buffer tube! ) would give you both the sliding weights and a lighter 22oz total, speeding up cycling. I'd try that first.

Good luck!


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My site: Blowback9.com

I have two 9mm AR-15/M16 uppers.  Both receivers are Gibbz G9 pattern but one has LRBHO and the other does not.  I have two 9mm blowback bolts.  I don't recall both bolt manufacturers as one dates back to 2008, while the other is from 2017.  In M16 9mm full auto operation I use a Sylvan Arms Glock mag adapter or a VM-Hytech Uzi mag adapter, neither of which have LRBHO capability.   One of the bolts performs exactly as you have described in both uppers. The other bolt works perfectly in both uppers.  Both bolts work perfectly in semi auto in both uppers.  I tried numerous buffer/recoil spring combinations to try to get both bolts to work in full auto in both uppers without success.  The 2008 bolt that fails in the Gibbz pattern uppers used to work perfectly in a mil spec upper receiver (I swapped that receiver out for the Gibbz pattern upper receiver in 2017).

After inspection, it appeared that the 2008 bolt that fails in full auto is very slightly larger in diameter than the 2017 bolt that works in full auto in both Gibbz pattern uppers.  Drag marks on the failing bolt but not on the working bolt was the clue.  As a result, I now have the presumed larger diameter 2008 bolt in an AR9 semi auto carbine with the LRBHO upper where it works perfectly (KynSHOT RB5007 buffer, so no spacer needed).  And the 2017 bolt that works in full auto resides in the other (non-LRBHO) upper that I use for 9mm M16 full auto shooting (B & T AR9 AR15 buffer, and no spacer).

Disclaimer:  I have no affiliation with or financial interest in any vendor or manufacturer; and I am not a competitive shooter or a blogger who receives any form of compensation for endorsements or favorable public or private comments.  Use of vendor and/or product brand names, if any, is for informational purposes only.

Best of luck.
MHO, YMMV, etc.  Be well.

Edited for completeness and clarity.

Are you using a ramped bolt?  
One issue I ran into a long time ago was I had a ramped bolt that was aggressively ramped and when combined with a well worn hammer, the bolt wouldn't push the hammer down far enough for the auto sear to catch it on full auto.  
I was able to switch to a new hammer that wasn't as worn and that fixed the issue.

My first thought is you need sliding buffer weights.

Amphibian makes a great point about the hammer/bolt interface.

Have you function tested by hand in semi and full auto? In auto, you should be able to hear the sear drop the hammer as the bolt closes.

when we're building 9mm SMGs,(and assuming that we're running a collapsible CAR stock assembly) we use:
1.  M16 trigger group and auto sear
2.  standard CAR action spring
3.  9mm CAR-length 2-piece mechanical buffer that weighs in at 5.8 ounces
4.  Ramped 9mm bolt assembly weighing almost exactly 16 ounces with the "key" attached and weight installed.

Steve/RRA

I successfully used a virtually identical configuration with my M16 from 2013 till this spring, right down to the RRA 9mm CAR-length 2-piece mechanical buffer, with the possible exception that I have a 16" barrel.  I only changed to the B & T hydraulic buffer to get a significantly lower ROF while maintaining reliable operation.

Disclaimer:  I have no affiliation with or financial interest in any vendor or manufacturer; and I am not a competitive shooter or a blogger who receives any form of compensation for endorsements or favorable public or private comments.  Use of vendor and/or product brand names, if any, is for informational purposes only.

Best of luck.
MHO, YMMV, etc.  Be well.

FWIW, as mentioned in other threads, do not use a Geissele SSF trigger in a 9mm M16.  The Geissele triggers are for 5.56 only.  In a 9mm the hammer will peen and flatten and eventually cause function problems.

I agree for a straight blowback 9mm setup....but I use my Geissele SSF in my CMMG RDB 9mm, 40SW and 45ACP.  Not sure if officially supported or even tested by Geissele but I would say that my delayed blowback setups have a softer recoil impulse than a typical carbine gassed 5.56 upper w/ an H2 buffer and standard spring.

Very good point, I always forget about stuff I don’t own.

For the last 20 years, I have been running the two piece type buffers in all my full auto AR's.  Never any bolt carrier bounce, regardless of caliber.

Tony

Thanks everyone.
Lots to process.
Now I just need the time away from work to do it.

Yep, I had an issue with a PSA chamber that I had to touch up a bit with a finishing reamer. Ran like a top after that.