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Posted: 7/18/2020 4:43:47 PM EDT
I speed read through these 24+ pages and didn't see an mention of the failure pictured. I'd say this occurred after 1500 rounds or so. Gun continued to function and I discovered the failure during cleaning. Pictures sent to Bryan at CMMG. Nothing unusual at all during range session; ammo 145gr RN with a mild load probably making 125PF. Anyone speculate on what sort of stress might cause this? Wear on the Cam Pin became apparent very early in the gun's use; don't know whether this is unusual or not. Gun lubed every 200 rounds or so.
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Posted: 7/18/2020 6:17:24 PM EDT
[#1]
thanks for the picture... I don't have that many rounds on my radial bolt, probably 1000, but will keep on the lookout
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Posted: 7/18/2020 7:17:03 PM EDT
[#2]
Mentioned it in my video some time ago...
 CMMG RDB Excessive Headspace |
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Posted: 7/20/2020 3:11:43 PM EDT
[#3]
Could have simply been a bad part
Replace and move on |
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Posted: 7/24/2020 6:52:51 AM EDT
[#4]
Quote HistoryQuoted: Could have simply been a bad part Replace and move on Agree in principle but a)it's and expensive upper, b)low round count, c)had a firing pin mushroom at an even lower round count and d)this level of part is not readily available on their website. CMMG will make it right, but I'm going to have to ensure I have spares at the ready. I have two uppers, one for competition and one for home defense so I can't afford down time. With my limited AR-like platform experience I wasn't expecting to have to have a spare bolt on hand, but now I know better. Once 300 BLK ammo becomes a bit more available I'm going to consider switching to this caliber for HD. |
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Posted: 7/24/2020 8:13:05 AM EDT
[#5]
I would think the RDB system puts more load on the can pin than DI.
You are driving the whole mass of the system through a side load on the cam pin, whereas with DI, it is there to unlock and extract the bolt, well after chamber pressure has dropped. |
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Posted: 7/25/2020 6:37:33 PM EDT
[#6]
I can't tell from your pic... is there wear on the bolt lugs ?
I am not completely familiar with the lugs... But shouldn't those have wear marks ? If there is wear markings ... could you post a pic of that ? |
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Posted: 7/25/2020 9:05:01 PM EDT
[#7]
What’s going on with the bolt?
Looks like it has a chip or two missing 
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Posted: 7/25/2020 9:19:59 PM EDT
[#8]
No wear marks. In my OP I noted that I had seen a lot of wear on the cam pin itself. I don't know if that's normal, but the round count was very low
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Posted: 7/25/2020 11:42:03 PM EDT
[#9]
Looking at the point of failure, the grain structure of the metal doesn't look right to me. Looks like a bad part. Those things happen, not all defects are visible in a simple inspection.
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Posted: 7/26/2020 10:03:04 AM EDT
[#10]
Quote HistoryQuoted: What’s going on with the bolt? Looks like it has a chip or two missing https://www.AR15.Com/media/mediaFiles/455572/72120F87-BCE7-4589-B6AA-65021E3071F6-1519830.jpg Yep, most likely happened as soon as the cam pin broke |
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Posted: 7/26/2020 3:59:09 PM EDT
[#11]
Quote HistoryQuoted: Looking at the point of failure, the grain structure of the metal doesn't look right to me. Looks like a bad part. Those things happen, not all defects are visible in a simple inspection. I mentioned that in my original email to technical support. CMMG will get replacement parts on the way next week no doubt. |
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Posted: 8/9/2020 6:38:33 PM EDT
[#12]
A second Cam Pin (near) failure has occurred. When the original Bolt/Cam Pin assembly failed I swapped in a complete Carrier assembly from my new upper. After what I estimate is 1000 rounds, 1500 on the outside, the Cam Pin is worn as pictured. I had one stretch of 450 rounds during a match without relube, but while the first Cam Pin that failed showed surface wear, this second part is actually flatted. I'm going to continue to run it until I hear back from CMMG, but this is not a good sign. Any and all observations/suggestions welcome.
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Posted: 8/10/2020 7:12:54 AM EDT
[#13]
Looks like it's gauling against the carrier, is that damage on the side if the pin facing the front or rear?
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Posted: 8/10/2020 7:13:40 AM EDT
[#14]
If you look at the video I posted above, I think your barrel probably has excessive headspacing which is what I had with my original barrel. It was eating parts.
As mentioned in my video, I think that if remove the extractor from the bolt and fire one round with no suppressor mounted and if it extracts the case, then your headspacing is excessive. I also have more details documented at the bottom of this link: http://www.c3junkie.com/?page_id=221 |
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Posted: 8/10/2020 7:14:36 AM EDT
[#15]
Quote HistoryQuoted: Looks like it's gauling against the carrier, is that damage on the side if the pin facing the front or rear? It's on both sides (symmetric). |
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Posted: 8/13/2020 2:58:52 PM EDT
[#16]
You can clearly see that it failed along drilled hole intrusion.
And the interior of the hole itself looks pretty rough on the inside. Keep an eye on the gouging and wear on the end of the replacement part. Maybe a poor heat-treat or the material just can't handle the cyclic pounding. Might be worth it to have a few spares cryo-treated or NIB coated to see if there is an improvement. I would say that this is going to be part of the process for a period of time as CMMG develops better revisions in tolerances, shapes, treatments and materials. I'm chomping at the bit to build a few of these RDBS uppers and move away from blow-back systems, but waiting to see how it will progress with long-term reliability. I don't want to have a gun fail at a match. . |
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Posted: 8/13/2020 5:06:45 PM EDT
[#17]
Quote HistoryQuoted: If you look at the video I posted above, I think your barrel probably has excessive headspacing which is what I had with my original barrel. It was eating parts. As mentioned in my video, I think that if remove the extractor from the bolt and fire one round with no suppressor mounted and if it extracts the case, then your headspacing is excessive. I also have more details documented at the bottom of this link: http://www.c3junkie.com/?page_id=221 I'll definitely try that next week if not tomorrow. I do have 4 cam pins on the way from CMMG and I can see sending the upper back for analysis; I'd start putting rounds through the slightly newer upper if I had a complete carrier assembly. The cam pin currently in the match upper is from my a very early s/n 9mm guard; should be interesting to see if that cam pin flat spots as it worked fine in the old upper. |
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Posted: 8/14/2020 12:18:29 PM EDT
[#18]
Nevermind
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Posted: 8/14/2020 1:20:05 PM EDT
[#19]
Quote HistoryQuoted: EDIT: Shit, maybe I don't know as much about this as I thought. This weapon isn't even locked-breach? What's with the angled locking surfaces? Is this some kind of retarded-blowback design? Yep, radially delayed blowback. Still a dual-mass system The case drives the bolt rearward, which causes it to rotate due to the sloped lugs. The rotating bolt rotates the cam pin, driving the carrier to the rear due to the proprietary cam slot. So you are driving the whole bolt mass tangetially through the cam pin. On a DI or piston gun it's just there for locking/unlocking and extraction forces. To your initial point, in DI, the pressure behind the bolt reduces the friction on the locking lugs during unlocking vs piston. |
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Posted: 8/14/2020 2:50:27 PM EDT
[#20]
Quote HistoryQuoted: If you look at the video I posted above, I think your barrel probably has excessive headspacing which is what I had with my original barrel. It was eating parts. As mentioned in my video, I think that if remove the extractor from the bolt and fire one round with no suppressor mounted and if it extracts the case, then your headspacing is excessive. I also have more details documented at the bottom of this link: http://www.c3junkie.com/?page_id=221 Update: I tried the experiment, above and the results are as expected (almost) with the extractor removed ' Test 1: Problem upper with usual lower (TriggerTech trigger @ 2 lbs TPW) - without the extractor the round wouldn't even fire while this combination worked reliably with the extractor in place. Don't know if this is related to headspace. Test 2: Problem upper with lower #2 (Mil trigger) - round fired and case ejected; would seem to indicate a headspace problem. Test 3: Second Upper with the TriggerTech lower - fired and did not eject the case. Appears that headspace is likely an issue. I'm going to relay this information to CMMG. |
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Posted: 8/14/2020 6:37:06 PM EDT
[#21]
Quote HistoryQuoted: Update: I tried the experiment, above and the results are as expected (almost) with the extractor removed ' Test 1: Problem upper with usual lower (TriggerTech trigger @ 2 lbs TPW) - without the extractor the round wouldn't even fire while this combination worked reliably with the extractor in place. Don't know if this is related to headspace. Test 2: Problem upper with lower #2 (Mil trigger) - round fired and case ejected; would seem to indicate a headspace problem. Test 3: Second Upper with the TriggerTech lower - fired and did not eject the case. Appears that headspace is likely an issue. I'm going to relay this information to CMMG. Regardless Test 2 and 3 sound similar to what has happened to me. |
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Posted: 8/15/2020 1:38:31 PM EDT
[#22]
I'm not certain of my test validity. The gun was functioning 100% before the test. Once the extractor was reinstalled I'm experiencing ejection issues similar to my early symptoms related to ejector springs. I'm going to diagnose this afternoon, but expect that the ejector spring has failed and I don't know when during that 3-step test sequence the failure might have happened.
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Posted: 8/15/2020 2:55:32 PM EDT
[#23]
Quote HistoryQuoted: Test 1 doesn't make sense. Don't understand why it wouldn't fire with no extractor installed. Same ammo used in test 1 and 2? Regardless Test 2 and 3 sound similar to what has happened to me. If it has excess headspace, then the cartridge can sit deeper into the chamber as long as the extractor isn't holding the rim and keeping the cartridge against the bolt face. In this case there won't be enough firing pin protrusion to reliably ignite the primers. |
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Posted: 8/15/2020 3:20:28 PM EDT
[#24]
Quote HistoryQuoted: If it has excess headspace, then the cartridge can sit deeper into the chamber as long as the extractor isn't holding the rim and keeping the cartridge against the bolt face. In this case there won't be enough firing pin protrusion to reliably ignite the primers. |
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