This is more in par with what I am getting out of mine. Thanks for sharing.

For those of you who are owners of the 5" or 8" CMMG RDB setup and have put some decent number of rounds through it, would you recommend it to someone looking at the platform now? Is there any reason to hold off and wait?

Thanks @j4c3 for posting up another data point.

I was unexpectedly able to get away to the range this evening with mine again. Previously I had only tested my accuracy at 20yrds because that is the zero distance I use for competition. Well I had a theory that possibly the bullets hadn't fully stabilized at that distance yet and the groups wouldn't open up more if I moved the distance out futher. Well the short and sweet of it is I completely disproved that theory. The following is the range report.

First of all I was shooting from a solid concrete bench. I put the magpul bipod back on the rifle, was supporting the rear with a sand bag and even switched in a Geissele SSA-E trigger in place of the G2S I had been using. Trying to eek out every last bit of potential shooter error.

First group I shot at 50 yards was S&B 124 with the Holosun HS510c 2moa dot with a Holosun 3x magnifier behind it.
2.6" or 4.95moa


Next group I shot at 50 yards was Federal HST 147gr with the Holosun HS510c + 3x magnifier.
3.19" or 6.09moa


I also brought along my 16" 5.56 AR with a Leupold VX-R 3-9x40 scope that shoots 1 moa or less for me with factory Frontier 75gr ammo.
At this point I removed the Leupold scope from the 5.56 rifle and installed it on the CMMG 8" guard build.

That's some weird looking stuff right there.

Went to sight it in on another target and as I did. The skies decided to open up and let all the rain out. Probably rained for a solid 20 mins. Once that was over I could get on with shooting groups.

Next group I shot at 50 yards was S&B 124 using the Leupold VX-R scope set at 9x magnification.
3.89" or 7.42moa


The final group I shot, out of the CMMG guard build, at 50 yards was Federal HST 147gr using the Leupold VX-R scope set at 9x magnification.
2.99" or 5.71moa


None of those groups were good or even significantly better than any other groups I've shot with this CMMG guard build. In fact the group sizes were exactly in line with the groups sizes I was seeing on my second range trip when I was using just the red dot unaided by any magnification. Really disappointing. Especially since the manufacturer is trying to sluff this off and say this is acceptable accuracy from this system.

Well before I could leave I needed to remount the Leupold VX-R on my 5.56 rifle and reconfirm zero. That way I know it is good to go. Also a good way to tell how well I was shooting today.
Heres the re-zero group I shot at 50 yards out of the 5.56 AR with the Leupold VX-R set at 9x and using Frontier 75gr factory loads. Used the same bench, magpul bipod and sandbag.
0.5" or 0.95moa

Here's the same group with out the range buddy marks all over it so you can more clearly see it.

So again definitely not the shooter that is the issue here.

I've started talking with a gunsmith who specializes in accurizing the AR platform. Both pistol and full length stuff. He makes some pretty solid claims about his accurizing. But since CMMG has basically left me in the dust and said they don't see a problem here I'm trying to decide what to do. I'm definitely going to keep pushing on CMMG to see if I can get anything. But I'm also trying to decide if I should send it off for accurizing along with just a complete rebuild with a new receiver and quality geissele handguard. Or dump it all together as my PCC Competition gun and go get a JP, MBX blowback or even a Sig MPX.

Anyones thoughts are appreciated.

@styer which gen endomags are you using?

I was having rounds pushed into the case when chambering on gen 1 endomags. COAL's were all over the place after chambering, visually I could see they shrank. The gen 2 endomags have a better feed ramp that eliminated the problem for me.

I have the Gen 2 endomags. I actually tested that theory as well. Loaded up 5 rounds in a mag and then let the BCG release from locked postion to chamber a round. Then pulled back the BCG to eject the round and locked back the BCG again. Repeated this for all 5 rounds. I measured each round before and after with calipers and kept them in order. Every round was within 0.001" of original measurements.

I've actually gone ahead and ordered a brand new Aero Precision XL upper receiver and an Aero Precision 7" Quantum m-lok handguard. My last and final theory is that since the handguard and receiver I'm currently using were reused from a previous build. There may be some issue with them that I can't easily see. The handguard was a cheap one which doesn't give me 100% confidence in its barrel nut even though it seemed to install just fine. After this if it's not right and consistently shooting under 4-5moa then the problem squarely rests with the CMMG parts. And I'll either send it off to the gunsmith I've been talking to or just dump it and be done.

Sucks you're having issues.  I've definitely noticed my barrels like some ammo more than others.  Since I reload only 147Gr for 9mm.  I'm just looking for decent accuracy with my reloads and it is good enough for me.  If you've read my posts, you've seen that I have two 8" CMMG RDB builds.  I think that has helped in getting them to help me.  As I had ejector spring failures, then learned they initially shipped with 556 ejector springs then later changed to the same ones they use in their 45 ACP and now 40SW bolts.  Likewise, since I had a second BCG, noticed they changed extractors too.  Changed barrel extensions and how deep they ream them in the earlier vs later barrels.

Sucks to be the guinea pig but I still think it is the best closed bolt PCC / SMG out there right now due to all the aftermarket support since it is just the BBL and BCG that are proprietary.  
That said think about how many AR barrels makers are out there.  How many are top shelf and how many are crap.  Anyone with the 9mm headspace gauges and familiar with turning barrels for the AR should be able to make you a Guard barrel from a good blank.
Off the top my head, I would think Mad Machinist should be able to help you since he does the .45 gas operated uppers and makes 9mm barrels.  Should be easy for him.

I wouldn't give up on the CMMG RDB just yet.  Any straight blowback AR will not be as smooth.
I have an MPX and never shoot it now that I have the CMMG RDB.  My lower is more ambi than the MPX and I use the same trigger in CMMG RDB as my 556.  You may not care about suppression but it sucks on the MPX.

I know you have thousands of rounds through your CMMG RDB setup. For someone looking at it, would you recommend it? Or recommend waiting a little longer for CMMG to work out the last kinks?

Yes, I would still recommend it.
However, if you look at my website: http://c3junkie.com/?page_id=538
You will see that I have 4 specific goals.
One of them is the full auto cyclic rate in the 600's.  Most people on here don't care about that.
My buffer/buffer tube and spring selections achieve that rate but as a byproduct helps w/ the flatness/softness which many do care about.  
Lastly is a good suppressor host.  It does great with that out of the box or with the CMMG recommended components.  However, IMHO, it still lacks behind the MP5 in terms of smoothness when suppressed unless you change the bolt angle geometry like I did....but that is to an extent that I think 95% of the people out there won't ever notice.  Most people don't care about beating a full auto MP5 suppressed.  
Messing with the bolt angles and even just using my recommended buffer/buffer tube and spring selection seem to shorten the life of the ejector spring in this platform.
Going to the fixed ejector has fixed that for me which I know is too much trouble for most people but again most people don't care about my 4 goals.  Just some of them if any.

The key here is the flexibility/modularity of this mechanism.  Nothing else can touch it at this time.  
Now I don't think CMMG is perfect and they may be using some substandard parts...firing pins (I broke 3 so far).  Maybe barrel blanks (although I haven't had severe issues as the other poster here).  I've heard several people complain about their uppers receivers being out of spec but I never bought one of their upper receivers so have no first hand knowledge on that.

I think it can satisfy most users and especially those that are not 'tired' of the AR platform.  
I think a lot of people just don't like AR's anymore which I think is strange since we are on AR15.com.  I see a lot of posts where people are not interested unless it does away with the buffer tube and go to a side charger.  Which to me kills so many great options.  Options that I want.  However, as we've seen here even if you want to get away from the buffer tube and go to a side charger or whatever, there are aftermarket products to do that with the CMMG RDB if you really wanted to.

So I'll break it down to the type of customer:
1.  Someone that hates to tinker and just wants to yank a trigger but wants smoother than a blowback AR9 and wants the highest level of reliability.  
A.  Get CMMG BCG/BBL and use standard spring / buffer
2.  Someone that hates to tinker but now realizes not as nice as an MP5  
A.  Use my recommended buffer, buffer tube and spring...but must be willing  to keep an eye on the ejector spring life as I mention on my site.
3.   Someone that loves to tinker and do anything to try to beat the MP5 and have the ergonomics of the AR platform
A.   Fabricate a fixed ejector and keep experimenting.....

I personally don't see anything on the market right now to touch it even with some its shortcomings.  That doesn't mean there is not market for competition.  
Still waiting on a roller delayed blowback for the AR/M16 family but still uses the buffer tube to take advantage of the flat wire springs and current/future buffer options....but for now, this is the next best thing.

One last thing.  I had my recommended Kynshot RB5007 break where the head of it fell off.  Kynshot replaced it for their improved version where you don't see the pin on the head.  I didn't mention it because I thought the failure may have been induced by my spring loaded tungsten weight impacting it.  Another member here contacted me that theirs broke too and it was the current design which is concerning to me.  
As mentioned, I'm running my setup w/ the belt fed Shrike and many other configurations outside of the CMMG RDB and it has done wonders for everything.  Very happy with that buffer and willing to keep using it as the performance gains are worth it to me.  Having one fail on me as well as hearing about someone else having the same failure, I will agree it is not as robust as an 'encapsulated' buffer design but I'm not kicking down doors either.
I've had MGI, AAC, Griffin Armament, Colt Hydraulic and other buffers fail on me where a pin sheers off, polymer components fail etc...the key here again is the flexibility/modularity of the AR platform so that if any component is to fail (or if you want an 'improved' version of that component) you can replace with another vendors component.

Yeah I haven't given up on it completely yet. I totally agree with you on the aftermarket support for this rifle is untouchable. My experience with frankenbuild  blowback 9ARs hasnt been good in the past so I really don't want to go that route again. The reliability on the CMMG has just been absolutely great. But at some point I wonder if how much I've spent on this build could have been put towards a factory built competition 9mm gun. And just had it work well right out of the box. As it stands I've got a brand new quality upper receiver and handguard on the way to try. I may also give Rudy a call tomorrow and see if he is willing to do a barrel for this system at all.

@anphibian I also appreciate all the leg work you've done with this system. One of the main reasons I went this route. The ability to run same trigger and endomags to make it the same as my main AR is wonderful.

Yes, but I personally do not think any blowback AR can be as smooth.  So sure it will 'work'.  Just like a Honda civic can get you from point A to B just as good as an Acura NSX can.  Both are good cars and reliable but one is built for performance.  
I think you also state that they CMMG RDB does 'work' it is just accuracy you are not happy with.

I shot a SMG match today.  Put down close to 500 rounds pulling singles and a lot of full auto bursts.  I had zero malfunctions.  Ran great.  Very pleased with and accuracy was great.  The match had max distance today of about 40 yards.  
Let us know what Rudy says.  I may have to dig up some high end 9mm blanks.
Sure, hope you don't give up as it has been great for me.  As mentioned before, I think CMMG was quick to resolve some of the issues I had because I have two BCG/BBL sets to compare to.  Was thinking maybe you can see if anyone has a 8" CMMG RDB near you to see how the accuracy is so you don't have to drop more coin to get it resolved.  If you can show them that you get much better groups out of another one of their barrels, maybe they will swap it.

OK.  So starting to have FTE now....
Lost close to about a 1lb of strength and now having problems.
No change in 9mm ammo or 9mm host configuration.  What is interesting is that I was never encountering FTE's in 40SW during my testing of .40SW, it was always just failures to push the BCG into battery and now having FTE's in both calibers which I think is ejector spring failure.

@amphibian so about twice the life with the 40s&w bolt that has the rounded ejector. That seems like a great improvement but still at 1k rounds probably not an acceptable lifespan.

Also I talked to Shane, I believe it was, at Macon Armory this morning. He said he's got a CMMG 9mm RDB upper he hasn't even had a chance to attach to a lower yet. He also has a project on the shelf for another customer that wanted a 9mm barrel for the RDB system. He hasn't had much time to work on that project and when he feels that he has that design all dialed in he'd reach back out to me. If you're also interested I'd send him an email so he can see what the potential demand is.

Wow, this can't be coincidence because I started noticing the same issue now. Two weekends ago I experienced a single failure to eject on Walmart Winchester USA Forged garbage ammo. I chucked it to being the ammo. Then this past weekend I got 2 failures to eject on my reloads that had been flawless up until now, and which I competed with twice now without issues, from the same batch of reloaded ammo. Thankfully it wasn't during a match, it was rezeroing the upper and plinking with a friend. For reference, these same reloads had 0 failures in my Oly PCC upper for close to 3k rounds or so.

What's the remedy? A stronger ejector spring? Or is it the extractor spring that is the problem?

Edit: I have a carbine tube, Tubb flatwire .300 Blk lightweight spring, H Buffer

Yes, better but not good enough.  Mechanically slowing the bolt down by changing the angles sure does kill the spring faster.  
Yes, I'm interested.  Regarding the ejector and barrels see my next post responding to BeltFed_1

I don't know if you've followed my posts on the evolution I went through and my resorting to a fixed ejector which has resolved all my issues.
If not, this thread covers it: https://www.ar15.com/forums/armory/Full-Auto-CMMG-Guard-Tuning/23-494385/&page=3
As well as my site here: http://c3junkie.com/?page_id=538

As mentioned in those links, going to a fixed ejector has resolved all my issues but unfortunately this option is custom and not sold publicly.  I only did that .40SW bolt running 9mm testing since I was curious if those less aggressive bolt lug angles would work in 9mm suppressed.

With my fixed ejector setup it has been running 100% and for some reason my changing the bolt lug geometry slowing down the unlocking has killed my extractor spring.  My extractor spring is about completely dead.  I can literally shake the bolt and the extractor will flop around....but it is still running 100% so by that logic I don't think extractor spring means anything.  I even posted that I tested firing the CMMG RDB with the extractor completely removed and it worked for like 3 rounds and would have a FTE because it really just needs the extractor for the case to pivot on.  It isn't needed for 'extraction'.

I also posted how you can take a rod and put it down the bore and push on the bolt and feel it move a slight amount before the lugs engage the extension.  That slight air gap is what I think is killing the ejector springs.   Like being in a car hitting a brick wall at 1000MPH.

All that said, if someone were to turn a new barrel blank and head space it so there is NO gap I would think it would vastly increase the ejector spring life but I don't know what that would do in regards to the delay mechanism.  Both of my BCG/BBL combos have the gap so I'm thinking CMMG designed it that way.  I think the MP5 is the same way with a very slight movement and then you can feel the rollers engage.

Thanks I appreciate it. A little disappointed to hear about the issues some of you guys are dealing with. Maybe still teething pains for the RDB design as they work out the kinks. Lots of potential, just not sure if I want to jump in now or wait for it to get developed a little more.

Yeah, earlier I posted that if you wanted the highest level or reliability and not a tinkerer to just go with their BCG/BBL combo and use their recommended buffer/spring setup.  (standard carbine spring and standard buffer).
I say that because that is what CMMG will support and logically would think lasts the longest.  That is what they supply in their complete guns as well.  
All that said, I don't see many people post about that factory default configuration.  I'd assume there are a lot of them out there by now and strange I don't see many posts on it.  I'm just assuming it is problem free but I don't have anything to back that up.

I probably didn't fire more than 30 rounds in factory recommended configuration as I didn't like it...again comparing side by side the MP5.  However, again it is still better than any straight blowback AR9.

@Tranzformer my only issue has been with accuracy and I think I got a bad barrel after all my testing. I have only used the CMMG recommended setup with carbine spring and carbine buffer along with 9ARC mags and reliability has been absolutely 100% over the 600+ rounds I put through it in under a month.

An Update on my barrel:
Sent another email to CMMG yesterday morning with the details from Fridays range trip and it's gone completely ignored. At this point they have been ignoring my emails since last Tuesday. So this morning I contacted the retailer I purchased the kit from and was asking if they could contact someone else at CMMG on my behalf. Well they went ahead and offered to exchange out the barrel for me so I took them up on that. Sadly they did not have any 8" kits in stock but did have a 16" kit in stock so that's what I'll be working with this time around.

For additional reference in case it may help you, I went back to the range and measured my groups at 50 yards. This was with a Burris Fastfire III 3moa dot, and a front bench rest. I was getting 3" 5-shot groups with my 115gr FMJ reloads chronoed at 1320fps out of my ~11" lightweight reprofiled barrel.

Before I had gotten a 2.50" group with the same ammo.

Just another data point.

Thanks for commenting. PM Sent.

@CMMGinc I did get a reply.

Yeah, thanks for posting again.  You guys ghosted us for like the past 12 pages.

I'm curious to hear your thoughts on the relatively short ejector spring life on the 9mm RDB's.  I don't visit many other forums so much of my data is from people here and a lot of my own testing.  My theory is that the short life is because there is an 'air gap' between the back of the bolt lugs before it hits the lugs of the barrel extension.  That is the case in both of my 8" 9mm RDB builds.  As posted in this thread and this one: https://www.ar15.com/forums/Armory/Full-Auto-CMMG-Guard-Tuning/23-494385/?page=1
I have been trying various things and finally went to a fixed ejector that has resolved all my issues.

I think that if the headspacing was tighter so that there is no gap between the back of the bolt lugs and the extension lugs the ejector spring would last a long time but I don't know what that would do for the delaying and if it would be less smooth.

Don't know if CMMG got back to you yet but I took my second barrel out today and it is considerably more accurate than my first one.

Thats great to hear about your second barrel! Gives me hope!

I'm currently awaiting my new 16" barrel from Primary Arms. Who was nice enough to offer to exchange out my 8" kit that was having issues. New 16" kit shipped yesterday so hopefully I'll have it Monday or Tuesday. And it will promptly be built and brought to the range for testing as I have a match I'd like to attend next Sunday.

CMMG has also been more communicative now after the rep, here on ARFCOM, contacted me.

So I contacted CMMG via email and explained my Failures to Eject that just started happening close to 500 rounds or so on my 9mm barrel/BCG build. I described what my ejector spring looked like, and based solely on that, the rep sent me a couple of replacements via mail. I just got them today.

In the image below, from left to right, you have a standard unknown-brand 5.56 ejector spring unused for comparison, then the middle is the ejector spring that was in my RDB (notice how much shorter, and bent), and on the right is one of the two springs CMMG sent me as a replacement. Notice on the replacement how much taller, and the slight purple tone or paint on one side.

After I installed it, it definitely felt much stiffer than both the original spring, and the 5.56 spring. I have a backup RDB bolt, and this new spring is definitely stiffer than even that brand new unused bolt ejector spring. So I will be swapping that one too. It looks like CMMG might have gone to a different ejector spring after I bought my barrel/BCG kits around October 2018.

Thanks for the post!  As you probably know, I've spent a lot of time chasing down the ejector issues.  
Most of it I have documented here: http://c3junkie.com/?page_id=954

I just received a couple springs from CMMG last month.  They appear to be identical to what I've received before.  I just confirmed and they are .028" thick while a standard 556 is .025" thick.  Pretty much the same length as the others and not painted. .89 vs .91".  Standard 5.56 ejector spring is like around .92"



Can you check the length and thickness of one you just received?

So maybe they made yet more improvements in less than a month??

Do you have a way to measure the strength?
Below is a new CMMG spring.  Crude method but seems to work for me.  I'm just using a trigger pull gauge. Put the bolt in a vise and then put a punch between the gauge sensor and ejector and pull down till the ejector is flush with the bolt face.  A standard 556 ejector spring brand new seems to be about high 6 to 7 lb range and once it gets to low 5lb and under FTE's start to happen.


BTW, are you running suppressed as well?
Are you using any bolt weight?
Ammo?  I know you mentioned some cheap Winchester Forged ammo + reloads before....what is the majority of ammo?  If reloads, what weight projectile?  Powder?

Sorry for all the questions, just seems like don't get much data for these scenarios.  I seem to be the only one posting about the ejector spring failures.  So I presumed it wasn't an issue for 'most' people.

I'm curious to hear how long the replacement works for you.  I think this is the Achilles heel of this system and it really needs a fixed ejector for heavy use....but if you're saying you only got 500 rounds before having FTE's, I don't think that is heavy use....

I'm starting to wonder if the variable is like I mentioned before and has to do with the headspacing in the barrel and bolt.  I suspect that if the gap is eliminated in how much the bolt can move before it engages the lugs of the extension the ejector spring life wouldn't be an issue.

My fixed ejector setup setup is still working like a champ but obviously not for everyone.  Hopefully, CMMG will come up with a fix to resolve it for everyone.

I have read all of your information on the ejector stuff on your website, I thought I would share to help you maintain more data points. There is a definite color difference between the old and new spring.

I have a micrometer, but I do not have a trigger gauge unfortunately. I'll get the data and answer your remaining questions here shortly when I can get time to take the measurements.

On a side note, today I had a local USPSA match, and I took out the RDB ~11" SBR with 9ARC inserts. I took first place overall. Not a single malfunction with the new ejector spring. Short round-count stages, so I didn't have to reload. However, one stage I brought out the PMAG mag coupler because the round count was close to 30, and if I had to make up shots, I'd be at a disadvantage, but less so with the coupler. I barney'ed it (+1 in the chamber) so I had 31 rounds ready. I went to bolt lockback on my last shot of the stage. We need 40-rounders!!!!!!!!!

In the match today, I shot about 200 rounds of my reloads, which is the typical diet. 4.5-4.6gr Titegroup, Xtreme Bullets 115gr FMJ, not their plated stuff, FMJ. Not sure where they source or used to source their FMJ 9mm bullets.

Edit: I take the part back about the color. The springs are the same/similar color except for the purple paint that is visible only on one side of the new springs.

@amphibian

About my CMMG Guard barrel/BCG build:
16" barrel/BCG kit
ADCO cut to ~11.3"
CMMG enlarged port stripped upper receiver
Colt 901 lower
Tubb's .300 Blk lightweight flatwire buffer spring
H Buffer
No weights in the carrier
9ARC mags

Although this is my current configuration, when I first put it together I messed around with different buffer weights, but not very large round counts. 25 rounds at the most.
Upper has seen mainly my 115gr reloads: 4.6gr Titegroup, 115gr Xtreme Bullets FMJ (not plated). About 400-450 rounds of this stuff, maybe more.
However, I was developing a 147gr subsonic load that I shot through this upper for chrono data. So it has seen some 147s while developing the load, with the Hybrid suppressor on, and at the same time the 115gr reloads with the can for comparison. Suppressor use has been very little, but it has had some. If I had to guess, maybe 50 rounds at the most?
I have also shot a box of 150 rounds of Winchester USA Forged 115gr and about a magazine worth of Federal 115gr aluminum cased stuff.

I am running out of my 115gr FMJ reloads, so I will be transitioning unfortunately to some plated Round Nose from Xtreme Bullets (not very accurate), 4.6gr Titegroup for 115s and 3.5gr for 147s. The 147s were the subsonics I was developing before.

The way they cut these springs does not make it easy to get consistent thickness measurements.
Ejector spring measurements:

New spring (purple tint)
Wire thickness .0270" +/- .0005"
Length .909"

Old spring
Wire thickness .0270" to .0275"
Length .828"

Backup bolt assembly spring (unused)
Wire thickness .0275" +/- .0005"
Length .841"

Notice how my backup bolt assembly ejector spring is very similar in length to the failing one. The new springs are clearly longer. Like I mentioned before, both my bolts were originally bought around October 2018.
I have no way to test for strength, as I stated before, but I will tell you that the new spring is stronger than both my unused backup bolt assembly ejector spring and the spring that was failing.

I will keep an eye on round count and ejector spring life going forward.

I hope I answered all your questions. Let me know if I missed some or if you have new ones.

Thanks for all the details.  Please keep us updated.

I had the same problem. The ramp on the gen 2 are way better in my opinion.

Y'all should try some HK416 ejector springs.

The HK folks assure me they have teutonic space magic, and I have heard claims of great endurance from knowledgeable folk.

Perhaps a titanium ejector would reduce mass, also.  Just a thought.  Would be easy to make.

Hmmm...do you or anyone else know if the HK416 ejector spring will fit in a standard AR bolt?  Or is it too fat?  Pretty cool that it is braided wire and I would think it would definitely last longer if it fits...but wow...$30 for a spring!

Titanium ejector was discussed here: https://www.ar15.com/forums/armory/Full-Auto-CMMG-Guard-Tuning/23-494385/&page=3

I don't know if anyone tried it though.

Well I received my replacement 16" 9mm MkG builders kit from Primary Arms on Monday. Opened it and immediately found a lug on the bolt had a chip off of one of the front corners. Very disappointing. Both builders kits I have received have sustained damage to the bolt face during shipping while in their factory package. CMMG should put an evosote foam puck between the barrel and the bolt in it's plastic tube packaging to protect them both. Or at the least have the rear end of the BCG facing the chamber instead of the bolt face. Oh well that is neither here nor there as I contacted the Tech Rep at CMMG, I have been working with, and they happily shipped me out a new BCG the same day and I also mailed back the damaged one same day.


So fast forward to Wednesday I received the replacement BCG and was able to take the 16" build to the range to sight it in and do a little accuracy testing. Spoiler alert this barrel is a world of difference from my 8" barrel which I still believe was defective in some way. I won't be as wordy with this one or post as many pictures but if you care about results I will post those below this picture and also go into some of my thoughts about the results.

All groups were shot at 20 yards and groups were shot from left to right and top to bottom with the exception of the one in the "head" area. It was shot last. All groups were shot with my Holosun HS510c 2moa red dot. Some groups were shot with an aditional Holosun 3x magnifier and are marked as such.

MinuteMan Munitions Blue(synthetic) Bullet 125gr TC
1st Row - 1st Column: 1.37" or 6.53moa
2st Row - 1st Column: didn't measure but larger
3st Row - 1st Column: didn't measure but larger
4st Row - 1st Column: 1.13" or 5.39moa
This was a new factory load I wanted to try for competition since it was reasonably priced and has a bullet profile close to the Hornady XTP or Critical Duty just without a hollowpoint. Thought that might give me a better accuracy potential. Come to find out these barrels seem to just not like synthetic bullets at all. You'll see in the next loading. Also while this was at the upper end of acceptable and as good as the best groups I could produce with the previous 8" I think we can do much better.

Federal Syntech 150gr
1st Row - 2nd Column: 2.41" or 11.49moa
2nd Row - 2nd Column: not measured but not much better
For some reason it really looks like these MkG barrels just do not like the synthetic coated bullets. The 8" barrel I had would not shoot well period but groups for Federal Synthetic 150gr are very similar between the 16" barrel I have now and the previous 8" barrel. The 16" may just be marginally better but I did not shoot enough groups of this load to be able to really tell.

Federal HST 147gr
1st Row - 3rd Column: 2.34" or 11.18moa
No clue what happened here but I'm not going to shoot this load again. Maybe these barrels don't like the heavier loads. Just speculation here as this load shot better in the previous 8" barrel.

Hornady Critical Duty 135+P
2nd Row - 3rd Column: 0.7" or 3.35moa but if you take out the flier it would have been 4 shots through the same hole for 0.12" or 0.58moa!!!!!!!!
4th Row - 3rd Column: 0.59" or 2.83moa
Now this is what I was expecting to see out of these barrels! These groups are under half the size of the best group I shot with the 8" barrel using the same ammo. Keeping in mind this is expensive defensive ammo I would expect to see good groups with. I still want to find an inexpensive load that groups decently for use in competition.

Sellier & Bellot 124gr RN
3rd Row - 2nd Column: 0.6" or 2.85moa
WOW! While this factory load was one of the better shooting loads in the previous 8" barrel. It always came in above 6moa. This is a world of difference at 2.85moa and more than acceptable for a cheap factory load. Also helps confirm something wasn't right with the previous 8" barrel.

Fiocchi 9AP 115gr
3rd Row - 3rd Column: 0.64" or 3.08moa
"Head" Area: 0.8" or 3.83moa with NO magnification (shot to re-check zero after removing 3x and moving red dot back)
Again WOW! I was not expecting this very cheap load, I picked up one box of on Monday, to do well at all. But for $8.49 a box at a local gun shop I had to try it. Also shot some double taps off target with this versus the S&B 124 to see quickly which if either had more dot bounce. Personally they felt very similar in recoil and dot bounce in my quick and very non scientific test. So I ended up deciding on this Fiocchi 115 to be the loading I will use for competition in this rifle because I can readily find it local for cheap . Now I've gotta go back to that store and pick up a bunch of this stuff!

Overall from the testing I have done I think I can confidently say there had to be an issue with the original 8" barrel I received. I think this is clearly shown when you look at the groups from the Hornady Critical Duty and S&B. Groups were shot from the same exact boxes of ammo as previously with the 8" and the 16" is now producing groups less than half the size. While also noting it seems these barrels do not like synthetic jacketed bullets but they do like cheap lighter weight full metal jacket round nose factory loads. May also not like heavier loads but I can't say that with confidence.
All in all I am very happy with the results I got from the replacement 16" barrel I have now. Can't wait to take it out to the local IDPA PCC competition this weekend!

This thread now has me a bit worried, I never did accuracy check my Gen1 8" bolt/barrel combo (I was more interested in making sure everything worked FA).  I guess I need to check it out.  

@cbecker333 I don't think it's an inherient issue in their barrels. I now simply think I got a bad barrel previously. Quality issues do happen some times and can slip through the cracks. It's how the manufacture handles that afterwards that really matters.
I do think you should accuracy test your barrel but bet you'll get decent results.

I have the complete banshee version with the 5" barrel. I didn't measure groups but when I sighted it in they were comparable to my mp5k, a couple inches at 30 yards with a red dot. I shot 500 rounds the first time out, half semi auto and half full auto with no malfunctions. This was back in May. I'll be getting it out again soon and do some better accuracy testing also.

Attached File

Ok, I've finally made it through this entire thread along with many others...

First, many thanks to @amphibian and everyone else for the wealth of information.  It was a long read and I've probably forgotten as much as I've learned but it was a nice way to kill a weekend while stuck in a hotel on a business trip.  Luckily, I started making notes and writing down questions around page 10 so hopefully I've retained enough info.  Below is my list of parts I've put together based on the info in the thread.

Lower:
Aero M4E1 Lower
Larue MBT trigger
PA-10 buffer Tube (substituted for Vltor A5) - Been doing this for a while now on pistol builds.
Kynshot 9mm Buffer model RB5007
Tubbs AR-15 flat wire spring model ARBUF? Standard 556 or Lightweight 300BLK?
SBA3 or SBA4 Brace
Endomags - CMMG versions

Upper:
Aero M4E1 threaded upper
CMMG RDB Kit 8"....or 5"....
Handguard - Was thinking MI since I have a 9" already but I'll need 2 more for the other uppers
CH, Sight(s), Muzzle device- TBD

As I'm reading I see that some use the same lower setup for not just 9mm but 5.56 and 300BLK.  My goal is to setup a lower that I can SBR and then have 3 uppers in 9mm, 5.56 (shooting suppressed a bit of the time) and 300 BLK (suppressed shooting subs mostly) that will be a "matched set".  Any input or thoughts on how a single lower setup will work across the 3 calibers?  I expect it won't be perfect for all calibers but I'm hoping for something that works well in all configurations.  While I'd love to have the problem of tuning for full auto, sadly I don't see that in my future any time soon.

I plan to start out with a standard upper.  If I need, I can open up the port.  Just curious to those that have gone this route, do you anodize or cerakote afterwords or just roll with it?

As noted in my list, which Tubbs spring should I go with?  I've seen where some are using the standard 556 spring and then I see mention and pictures of the lightweight 300BLK spring.  Maybe I'll just pick up both and test them out.

I'll probably go with the 8" kit using a 10% code at Optics Planet (~$337).  Unless anyone knows of a better deal, please let me know.

Also, the price of the Kynshot is pretty steep.  I've seen where people said they found them at decent prices so if anyone has suggestions of places to check for deals or sales, it's greatly appreciated.

Sorry for the long post.  As mentioned, I've been taking jotting down notes and questions so I'm just puking it all up at once.  Thanks in advance for any comments and suggestions.

Accuracy isn't exactly impressive with my 8" barrel. I get about 4" groups at 50 yards using fiocchi, s&b, and magtech 115. My German mp5k-n is a tack driver with fiocchi, so it's slightly disappointing, but also a fraction of the cost.

Otherwise I'm impressed. I used an aero xl upper and mega wedge lock rail. It runs very well on a colt post sample lower with A5H2 buffer. Round count is over 1000, all full auto, about half suppressed with a dead air wolf 9sd. So far the ejector spring seems to be holding up fine.

Does anyone know of a dealer that has the 8" barrel + BCG in stock? Everyone seems to be out of stock at the moment. Even Optics Planet has their 'expected to ship within' time period listed under availability. Is CMMG making some changes to it before more of the barrel and BCG combos hit the market again?

That is disappointing to hear. What were the best drops that you have gotten with your CMMG RDB and what ammo were you using?

The CMMG RDB AR9 shouldn't be that bad accuracy wise. Seems more guys are having and noticing accuracy issues?

I think you can just use the ARBUF (556 Tubb flat spring) fine in all 3 calibers...but...300BLK is a big variable as that all comes down to gas system length and how it is ported now.  Basically, if you have a working 300BLK upper that works fine now on a lower you run 556 on, you should be fine with this setup.

I think you will be surprised and it should run very nicely in all 3 calibers (again presuming your 300BLK upper is already working fine).

I've been using the 300BLK spring just to drop my full auto cyclic down another 30 RPM.  Very hard for me to notice a difference in feel from the standard 556 Tubb flat spring vs the 300BLK version so I would just run the 556 Tubb flat spring in all 3 calibers so you don't have to mess with anything in the lower.

Bear in mind that all the Tubb flat springs have the same spring rate.  The only difference is the length.  So you could get the longest and cut coils if you wanted to.
Locked and Loaded hast it listed for $297.35 but also out of stock: https://lockedloaded.com/product/cmmg-barrel-wbolt-kit-9mm-8-rdb-black
For the Kynshot RB5007, the best price I got was when it was on sale at Botach for $89.  Looks like they have them now for $110 which is the best I've seen at this time: https://www.botach.com/kynshot-rb5007-hydraulic-recoil-buffers-9mm-collapsible-stock/

Regarding 8 vs 5", you may have seen my posts that I only have experience with the 8" versions and have been happy with them and that Tom Bowers posted about the 5" being loud with his cans as well as the CMMG Branded ones but gave no more details in regards to whether or not the carrier had weights, ammo etc...
I like the 8" as I have my 147 Gr reloads to be right under the speed of sound for a ~9" 3 lugged UZI barrel as I want max energy for knocking down steel plates but still be subsonic.

As I mentioned before, I have (2) 8" CMMG barrels. 1 is an early one that come w/ an M4 dual feed ramp extension and the second one had the single feed ramp designed to be fed by Glock mags.  I have a couple local friends with them and haven't heard any accuracy complaints.  I also know someone that is a member here that sent me their upper that wasn't running with the Gen1 Endomags....I ended up cutting dual feed ramps into his extension which fixed his issues but later we all realized that the Gen2 Endomags, made that mod unnecessary.

At that time, I was changing to RMR Reloading's 147Gr JRN projectiles which have a fatter ogive than Precision Delta's and they were all getting stuck in the 'later' barrels.

Apparently, my early barrel is fine.  On my second barrel, I had run a finishing reamer down the chamber of my second barrel and these other guys barrels.

It turns out that my second barrel is more accurate than my first with my reloads.  I'll have to reach out to these other guys to see if they have done any groups but I'm happy with mine.  Even my first barrel is not great but not bad enough for me to complain about.

They both definitely seem to like some ammo more than others but I think that is the case with a lot of barrels.

Not really related to the CMMG, but which brand/model finishing reamer do you use? I've been wanting to get one because I have a number of aftermarket handgun barrels I'm noticing that apparently weren't finish reamed and it's getting irritating. Getting rounds not chambering depending on ogive shape at OAL 1.150", which I wouldn't consider anything out of the ordinary. Just needing to increase the leade/throat, don't want to touch the chamber length obviously. A lot of mfg these days don't bother to finish ream their stuff apparently

Use an adjustable gas block on those other uppers, easier to tune for the single recoil system.

I opened up the ejection port on the 9mm upper, and just left it as is. Pic somewhere in this thread. It's too much effort for something so small imo. Maybe mask it off and touch it up with some krylon if it bothers you. It's probably more obvious on an anodized upper, but FDE isn't that noticeable. The aero XL threaded upper is another option, since you are going with the threaded rail system.

Yeah.  Not sure what the deal is if maybe some manufacturers are using old reamers and not sharpening/replacing them or just too short.  
My partner makes barrels for other manufacturers and reamed out one of my Guard barrels with a Manson finishing reamer.
I didn't feel like driving back out to the shop to bug him so I figured I'd just buy one from Brownells.  I bought a Clymer and to my surprise, it didn't do shit.  I ended up buying another Manson like my partner used from Brownells and it did the trick.

Outstanding.  Thanks again for the great info.

Unfortunately, lockedloaded.com is on my $#!@ list right now as I am not impressed with them at all.  I have a SAR9 pistol in limbo somewhere because UPS screwed up delivery to my FFL and even admitted their mistake and told me to have the shipper contact them.  Only thing I've heard from lockedloaded.com is they want another fee to ship again.  They refuse to answer emails or phone calls and do not return messages.  They always seem to have really good prices but their customer service is horrible.  At this point I'd warn anyone away from them but that's just my opinion.  Especially on a kit like this where some folks have reported issues where they needed to exchange something with the seller.  I'm sure CMMG would step up and take care of it but still...sometimes it's worth a few extra bucks to go with a re-seller that is reputable and has good customer service.

And, as a recent post after mine points out, everyone is out of stock anyways.

I'll keep an eye on Botach.  I actually have that link saved already as they were the best price I had found so far.

Thanks again

Yup, good point.  Adjustable gas blocks are definitely in my plans though I obviously didn't list it.  I have a couple already in my parts bins but I'm not sure who they are from (Oding works maybe).  I do know that they use set screws to make adjustments and then to lock down the adjustment screw.  Ideally though, I would like something that maybe had repeatable settings with something like positive clicks so I could say, tune for supers and subs and know how many clicks either way to adjust, much like dialing a scope.  I know I've seen folks recommend the Superlative Arms and Seekins blocks.  I haven't kept up with gas blocks lately so any suggestions are appreciated.

And yeah, the Aero XL is on my backup list.  I just wish they made it in the M4E1 styling.  I know it has nothing to do with function but I do like the looks of the M4E1 stuff so it would be nice be able to use the same upper for all three calibers.  Good point on just masking and rattle canning the edge if it bothers me.

Thanks for the input.

Thanks for the info! I was close to buying the Clymer, so it's a good thing I asked you. I'll be getting the Manson instead then. It'll be nice to have barrels that chamber everything I throw at them again

I definitely wouldn't, and didn't, accept 4" or 8moa at 50yards.
In my testing I had also found that these barrels are picky on what ammo they like. My two have definitely DISliked synthetic coated bullets of any weight. However I do think my first 8" barrel was defective and the new 16" replacement barrel I got has been excellent so far. Even with cheap FMJ factory loads.
In fact I went to the local IDPA PCC match today and placed 1st with the 16" setup. I could tell my shots were landing where I was intending them too.
Everything else about these rifles has been fantastic. I wouldn't let my initial accuracy issues scare you off. I would though test what ever you receive with a handful of diffetent loads to see how it does.