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CMMG Radial Delayed.....Unintentional Full-Auto?


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Has anyone with the CMMG Radial delayed blowback system had it go full-auto on them?  I experienced this recently with two different triggers.

 

First was with a CMC, it was a 2-stage drop-in unit with the flat trigger, was supposed to be 2lb takeup, and 2lb break.  The second stage/break feels MUCH lighter than 2lbs...so much so that I took it out of my 3-gun rifle because I couldn't ride the second stage without breaking it unintentionally, and went back to the Geissele G2S-E that Had in there.  So I basically treated the CMC like a single stage with a smooth pull-through and put it in an SBR lower to shoot with the CMMG system and 8-inch barrel.  But the other day during a PCC match I was getting doubles and figured I was bump-firing because the CMC's second stage was so light and since the trigger is flat maybe I wasn't letting up enough.

 

Switched that to a Rise RA-140 that I had as a spare.  But then earlier tonight at a PCC plate shoot, again I was getting doubles and triples at what felt like full-auto.  This time, if I just pulled lightly I was able to do one at a time, but then if I gave it a good pull and held back, it went full auto until I let up.  Kind of like a Steyr AUG;s two-stage on full-auto.  That made me think it was the trigger since I had an earlier RA-140 a few years back that was doing this because the disconnector spring had compressed and kinked...so my first thought was that.  But this is two different triggers in a row that were going full-auto, or so it seems.

 

I checked the CMMG bolt group, disassembled.  Have recently replaced the extractor spring and ejection was 100% as opposed to all the FTE's I was getting.  The BCG seemed to be fine, the extra spring was holding back the firing pin so it didn't seem like slam-firing.  And again, I was able to stop the full-auto bursts by letting up on trigger.  But just wondering if anyone else has had their CMMG RDB go full auto and traced it to something in the BCG itself.  The buffer is an Armaspec captured H1 equivalent.  I sighted in with this a while back and it worked fine, wasn't getting any full-auto then.  I use an H2 equivalent when I shoot my 16" version for PCC.

 

I'm about to try yet another trigger group...this time probably a Larue or maybe another Geissele unit.

Edited by MoRivera
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53 minutes ago, philmadxx said:

This happened to me with a Timney trigger - it wasn't made for PCC, it was for a standard AR.  There is something different about a PCC that can make this happen - although not consistently.  I have a non-PCC CMC trigger in one of my guns - no issues.  You might have to buy one specific to PCC.

Phil 

Yes but this is CMMG radial Delayed Blowback system, rather than your more typical straight-blowback AR9 type system.  Because of that, it is essentially interchangeable with an otherwise 5.56 lower receiver...including a 5.56-weighted buffer and trigger. Have used the same BCG in my main 16" PCC built around it as well with a Hiperfire trigger as well as with various other triggers that are not 9mm-specific but have also worked fine with blowback AR9's...such as the Rise RA-140, the aforementioned Hiperfire and Geissele models.

 

I know that for some PCC's, the harder/faster bolt in regular blowback AR9's was making the rear 'tip' of the hammer (if it had that) strike the back of the disconnector, but neither of the hammers so far in mine have had that feature.  I would have attributed just the CMC to bump-firing, but the fact that it still happened with the RA-140 right afterwards seemed too 'coincidental'.

 

So this morning I thoroughly inspected and cleaned the BCG...spring is fine, firing pin is fine and not getting stuck through the hole in the breech, so I think that trulls out slam-firing. The new extra-power extractor spring completely cured the FTE's from before that I mentioned.  I took the RA-140 apart and replaced the disconnector spring with a newer stronger one, and put an H2-equivalent buffer in.  Dry-cycling and firing the disconnector seems strong and the reset definite. It was a little softer before, so tmrw I'll run it through and see if it's been solved.  If it has, then I suspect it was the disconnector spring.  If not, then I may have to try some others.

Edited by MoRivera
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So update....

 

 

Replaced the disconnector spring with a very strong unit and today at the range had no full-auto anything and everything was controllable.  Fast shots, slow shots, slow trigger pulls, hard ones.

 

So it seems I solved it but I wonder if it's just a matter of time until this new disconnector spring fails and it happens again.  Had two FTE's early but it was 100% from there.

Edited by MoRivera
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