This older thread is archived so could not post in it....

https://www.ar15.com/forums/Armory/CMMG-RDB-9mm-Chipped-Bolt-Lug/48-511703/


I have only about 2200 rounds through mine and I've got chips on three bolt lugs.  Been having a lot of FTE and hangups with an 8" barrel, but none on a 16" barrel.  Have replaced the extractor and ejector springs several times and again, with 16" barrel it holds up fine, but with the 8" barrel once it starts to get hangups can't shoot more than 1-2 rounds before another.







Would like to hear from those in the older thread as two how they dealt with.  I have a 'repair kit' that has an extra bolt, but I feel like I should also contact CMMG and get a replacement.

Thanks.  I emailed their customer support with pics but maybe I'll look into that as well.

I can’t help you diagnose it, but I’d be curious to know what buffer weight/spring you’re running, caliber, ammo type (full power or lighter competition type loads), and whether suppressed.

Also please let us know what CMMG says.

Mostly H1 and H2 equivalents.  The long upper seems to like the H2.  9mm factory 115-gr like Blazer Brass.  Have run the shorty suppressed with an Omega 9k shooting 147 reloads from Fenix and Freedom.  That was pretty early on and things were running pretty smoothly.  It's really more recently that the short version starts to hang up and then it just can't run past 1-3 rounds, just shooting normal factory.

I replaced the bolt and will be doing some tests this wknd.  You can tell on both that just when racking and ejecting a dummy round dry, it just makes its way out rather than being thrown or knocked out, more often than not just trundling out the bottom.  Unlike the longer 5.56 cases that it swings/flicks out, it just pulls and kind of tilts the little 9mm case and relies on the velocity of the cycling for it to fall out the side when it hits its limit rearward.  That seems to me the 'fatal flaw' with this design relying on the 5.56-style push-rod ejector that just tilts the case out, rather than a fixed one that physically knocks it out.

ETA:
Update: went to the range with just the 8" upper with new replacement bolt.

Ran like a top.

More detail documented here: https://www.ar15.com/forums/industry/Chipped-bolt-lugs-on-CMMG-RDB/145-301279/?r=4023471&page=1&anc=4023471#i4023471

Thanks for the update. Hopefully this bolt lasts you a lot longer than 2k rounds.

Wonder if this was one of the bolts with excessive headspace. I could see the bolt getting a good bit of free travel before the lugs slamming into each other to be pretty bad.

I did a measurement of headspace comparing my 16" barrel to the 8" barrel, using the method of chambering a dummy round and then pushing down the bore with a rod, checking that distance of movement.

With the 16" I got just under 2mm of rearward movement.

With the 8" I got around 1.6mm of rearward movement....so it seemed actually less than the 16" somehow!  

So I don't know, some weird combination of things was going on with the 8" upper.  But for all I know it might just start up again once a few more hundred rounds are through, whereas with the 16" upper things stayed fine throughout the last few times.  Hard to tell if I've really 'fixed' anything more than temporary, frankly.

I would be curious if your 8" upper extracts a fired case with the extractor removed and no suppressor mounted as discussed on my website.  My video below shows the issue with my original 8" CMMG RDB barrel.  My second and replacement barrel don't have the issue anymore.

https://www.ar15.com/forums/Armory/CMMG-RDB-9mm-Chipped-Bolt-Lug/48-511703/

My old thread. They ended up sending me a replacement bcg and I sold it.

Yeah, I've watched and referred to your material a lot, been a huge help.

Again, based on yesterday's session with the new bolt, one would think that it's all fixed and runs like clockwork.  But then some time in, it might all go off the rails again.  That's how it was the last little PCC match I had running the 8" upper....first run through was okay until a mag change, then a double feed, cleared and ran the rest of the stage.  Second time through nothing but hangups after one shot and I had to stop without finishing.


I saw this as well and also referred to it.  Was comforting to see that at least I wasn't the only one that experienced it.  Frankly, I'm getting closer to just forgetting about the whole RDB thing and going back to my blowback setups which were dead-nuts reliable.  As mentioned, no problems of note with the 16" upper, but I feel like it's only a matter of time.

I'm wondering if people with chipped bolt lugs have out of spec carriers.

As seen in my picture below, the CMMG Guard (RDB) extensions all seem to be like a standard extension that are squared off.  
I'm thinking if the bolt was allowed to cam too far then the point of the angled lug would slam into the flat face of the extension lugs and you would have the issue of the lugs getting chipped.  



I just looked over 8 different bolts I use for my CMMG's RDB's.  Some were 5.45x39 bolts I modified as well as factory, 9mm, 40SW, 45ACP and 10mm bolts and none of them are chipped like yours.

If you look at both my as well as Yamaha's pics of bolts with chipped lugs, it's the first or second one on the bottom after the extractor, and then the third one as well for me.  Also on my new bolt, the wear is visibly apparent on those 'bottom' lugs as opposed to the others.  Don't know what the significance of that is.

When I dry fire on a closed bolt with nothing in the chamber, there is a tiny bit of upward movement of the BCG.  With a dummy round none, but a bit of forward movement.

New bolt after a 200-rd range session...




Although I don't know if that's finish wear on those lugs or brass/copper from contact with round in the magazine from stripping.  In terms of the BCG's play in the receiver, doesn't seem to be any more than my other AR-based guns in 5.56.

Good news!

CMMG got back to me and will be sending me another bolt, so at least I'll have an extra backup.  Much appreciated to them for responding.

Could it be as simple as a bad batch of heat treatment of the bolts? If the lugs were too hard, they would become brittle. Just a thought.

Scott

That's what it looks like to me as well.
I've had AR-15 9mm and .22 firing pins that experienced so-called "infant failure" breakage (round count under 1,000) where they were overly brittle, and a sear for an M11-380 that was overly soft, all due to improper heat treatment.  Even Six Sigma processes have some failures.

MHO, YMMV, etc.

I would be thankful if it's just that, and then these replacement bolts hold up fine.  Would hate to have to keep checking and replacing every 2K rounds or so.  I really just want to settle on this thing and be confident in its reliability.  With the combination of this as well as the extractor/ejector issues, it's been rocky but strangely pretty reliable in my 16" upper.

If it's because I got a bad unit before, then all the better that I've addressed it with replacements....I hope.  But again, their support so far, responding to this as well as ejector spring queries, has been great so good on them.

Thanks for the update, and please keep us posted on the long term durability

Also want to note on the ejector.  They sent me a new one when I requested earlier but also on the 'repair kit' bolt, they are now coming with the o-ring as well, probably to help withstand compression/kinking better.  In my original, it was only the spring and enter post.

That's not an ejector, that's an extractor.  I was reading over on Reddit that a common fix for extraction issues on these is to use the BCM improved extractor spring kit.  It comes with the spring O-ring and stalk.