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Posted: 5/31/2025 5:57:54 PM EDT
| I am a FFL/SOT and have been testing a Full Auto build with no luck. Fire control group works fine running as a 5.56 full auto. I have a HAN Colt insert (the one where you need to remove the bolt catch) Im pretty sure it's a rock river arms barrel and upper. I am using a Failzero 9mm BCG. multiple different buffer setups. right now im using a - K-SPEC Enhanced AR15 PCC 9.5 oz Buffer, carbine buffer tube. Fires perfectly fine semi. Full Auto results in one round fired and one round just chambered, no firing pin marks. running out of ideas on this. Also it's a Full Auto hammer no notch. |
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Do you have different buffers laying around ? Try them all. Is the bolt satisfying the auto sear ? Is it dropping the hammer too early ? Bolt bounce can happen easier to a unlocked action. Is anything binding the bolt. Is the upper correct for a 9 m.m. Those are just off the top of my head. |
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| I think I've ruled out the buffer, have a ton of them, flat springs, reg springs also. Always the same thing - click, one round comes out, chambers the next then nothing. 3 different bolts. The failzero says it full auto rated. Thinking something to do with the hammer? everything says this buffer should work -- K-SPEC Enhanced AR15 PCC 9.5 oz Buffer? though when I was using a different buffer I was getting hammer strikes so possible the buffer. Not many sources for full auto 9mm |
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| Sounds like it's not tripping the auto sear. Check the sear timing. Sear Timing |
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I personally don't care for the straight blowback 9mm M16 configuration. However none of the delayed blowback setups didn't exist 20+ years ago so I used to run them. Here is a link to my old 9mm M16 setup:https://c3junkie.com/?page_id=752 at that link I mentioned an issue I ran into which could be what is happening to you which is a worn hammer. |
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I have three 9mm AR BCGs. All have the steel carrier weight swapped out and replaced with a KAK tungsten carrier weight, and operate correctly in semi auto mode. With leaving all components the same except for only swapping BGGs, one works in full auto and the other two fail due to bolt bounce (only one to three rounds fire in full auto before failure). The BCG that works and one of the failure BCGs are small extractor units, and one of the failure BCGs is a large (standard AR extractor size) BCG as is the BCG that you have. The weight of the small extractor BCGs only vary by 0.1-0.2 oz. The BCG that works in full auto is the lighter one, and has a slightly smaller diameter as can be witnessed by rub marks on the side of the failing BCGs in places where the bluing is unworn on the successful BCG. I speculate that there is drag in the upper receiver that the failing BCGs have that might be impeding anti bounce buffering as the failing BCGs fail with a standard steel, two-piece 9mm buffer, several hydraulic buffers, and a tungsten powder buffer, all of which buffers provide full auto when used with the "good" BCG. I've tried all three BCGs in a couple of different 9mm uppers with the same results. Over last winter, I ground, filed, and polished one of the failure BCG carriers at the rub locations, but have not had the opportunity to test it, and I'm not sure when I'll be able to do so. Best of luck with your efforts. MHO, YMMV, etc. |
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Originally Posted By PatrickClayton: That actually sounds like my issue, do you have a hammer suggestion? |
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Originally Posted By PatrickClayton: Anyone reading this, I fixed my issue, it was the rock river arms upper, not full auto compatible. How is the upper not compatible with full auto? Seriously, I want to know where the upper interferes with full auto operations. |
| I have a friend that is a 07/SOT and he has a Quarter Circle 10 lower build. The parts are the QC10 Glock lower. Standard semi, full, safe lpk. An H2 buffer and carbine spring. Flattop 5.56 upper with a Wilson barrel and a CMMG bolt. The thing runs like a scalded dog. It does need a heavier buffer as it runs too fast but no OOB or misfeeds. |
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