Looking forward to hearing how it goes, my cmmg bolt is off being milled for RLL and I am just not liking the mean arms mags. The wasted space just offends me for some reason. So
this might be the solution!

Please document your round count on your 9mm CMMG RDB.  If you are like me and my friends, you will encounter ejector spring failures at around the 1K mark and get really frustrated.  If you don't know, I have ejector spring failures with the MEAN bearing delay upper as well.  I think all these delayed actions, should not be using ejector springs that are inline with the recoil.  The same spring is fine in 556 since the bolt is truly locked which isn't the case for the RDB or bearing delay.  
That is why I went through so much trouble to make my hybrid Dissent full auto configuration which is extremely tune-able and I can actually keep full auto bursts on target easier with my build than my full auto MP5.  It will also work with a RLL.  

Any update on function/accuracy? If you can, please to a group from a rest. Ugh, eagerly waiting for my barrel. Never thought I would get my suppressor before the barrel...

No updates.

Frankly, this is my lowest priority for shooting. I really don't have any practical use for it other than burning up what remains of my 9mm handloads. Were it not for the suppressor, I would have gotten rid of this build by now.

Considering you're the one who started this thread, I am surprised by your lack of enthusiasm. It seems like you've only had this set up for a few days and you already want to get rid of it. Did it just not live up to your expectations? I doubt anyone (that is a civilian) has a practical application for this firearm. I certainly dont. I am curious just because I am waiting on my barrel from BRT.

Fair call out. I really didn't want this about 5 minutes after I pulled the trigger on buying everything. Was a "Oh! I want one of those" things when I first saw it, without thinking thru any practical use cases. That's important for me because I'm in the process of downsizing *everything* I own. Poor decision making on my part.

Now I'm stuck with it (due to the suppressor*). It'll sit in the back of my safe until I either get rid of it or I can find  purpose for it.
*I'm also currently stuck with a 308 suppressor I have no use for. Another poor decision. Both of those decisions are kinda eating at me, at the moment.

That said, I am surprised at how pleasant it is to shoot and its more accurate than me. I might try to find a PCC match to shoot it in.

Now that is fucking sick.

Beautiful weapon there Amphibian. Seems NFA did away with their S-9 lowers. Quick check on the website turns up notta. Luckily I already have one, but I wanted to get another to build out a rifle for the wife. Wonder why they got rid of them?

I contacted NFA before I bought the XK9 that they make.  They told me that they are coming out with a Gen2 but it won't be till the end of the year.  They didn't tell me what was changing.  Seeing how everyone complains about the 'cheesy' ejector in the lower, I figured maybe they were going to change that.
But based on this thread: https://www.ar15.com/forums/ar-15/New-NFA-S-9-Lower--Scorpion-mags---Pics--review-and-build-in-progress-with-CMMG-RDB/15-754694/
That guy got his working with the CMMG RDB so I figured, if I wait for Gen2 maybe the changes will make it more difficult with the CMMG RDB so might as well buy this one since the thread above was 'successful'.  
I purchased 10 of the same mags from Prepper Gun shop like he did (all clear version like his).
I opened the package on 4 of the 10 and they all SUCK.  I test fired it and it was jam city.  I pulled the mag from the gun.  Just stripping a round off with my thumb, it would hesitate for a few seconds and then the next round would move up.  So obviously a mag problem.  I checked the other 3 mags and they all the same shit.  I thought maybe a COL issue.  So I tried factory hollowpoints and just trying to strip rounds off with my thumb it would hesitate as well.  Obviously not good enough for live fire.  
Since I opened the wrapper they wouldn't give me a refund which is bullshit.  I can exchange them but why.  To swap out for another shit one?   I did return the other 6 for a refund but was also charged re-stocking fees.
I'm not buying from them again.

So I borrowed a factory CZ 30 and 20 round mag from a friend and I will need to clearance the carrier to make those work.  I ordered some of the PSA AKV mags which people seem to rave about and got them today.  They are like the CZ mags in that I will need to clearance the carrier for those also.  I have uppers and carriers that have been clearanced to run unmodified AK mags like the KS47 stuff and those clear.  So now I need to mill my upper and carrier out some more to test the factory CZ or PSA AKV mags.  I did only buy two of the AKV mags this time and PSA literally put them both in saran wrap and a plastic mailer bag with no padding.

What is more concerning is that a friend of mine bought one of these XK9 lowers at the same time and assembled his in a straight blowback configuration which is what it is designed for and even with factory CZ mags, it is jam city.  He says it is feeding too low.  Which is really what you need to make the CMMG RDB work....the feed height needs to be lower than straight blowback but I have shit mags right now so I don't know.
So now my friend and I are wondering if these lowers are just whacked and don't work with either.  Have you shot yours?

That's strange Amphibian. I wonder if they changed the feed geometry or mag angle going from an S-9 to that XK9 lower? My RDB S-9 will only feed well with PGS mags. I have about 8 of them in total. I've tried Magpul and they were a no go for both my AR-V and my S9. The AKV mags were a close second but I had to shave the plastic off the feed lips all the way down to the metal reinforcement and even then they induced malfunctions while feeding my S-9 lower. The PGS mags were the only ones that worked without issue out of the box. I will also admit I don't shoot much in the way JHPs with it, but I did test about 2-3 mags worth to make sure it would feed them well. If my memory serves me well there were a few hiccups with upward facing cartridges causing FTFeeds. It hated small projectiles like Hornady Critical defense, but liked Speer GDs and HSTs as long as they were heavy 135gr-150gr.

I didn't notice anything cheesy about my ejector but the BHO is so simple it's kind of funny? I don't remember much pertaining to the ejector though as mine was pulled out when I put the CMMG guts in it. The BHO actuator is literally a piece of folded round metal held in by a small machine bolt and small flat piece of metal. It's chitzy as all hell but it does function without issue.

To be honest when I built my S-9 I built it using your advice in the post you linked. I built it using a PSA KS47 upper instead of Gibbz or other NFA upper.  I had some issue with the Bolt latch but I remedied it with some sandpaper work to the upper and to the posterior of the latch itself.

I also built another 5" RDB upper that was mated to a now disassembled PSA Colt SBRed lower that I opened the magwell up some to accept modified Sterling SMG mags. I stopped toying with it though because I couldn't get LRBHO to work, I had a lot of fun building it though and may rebuild it again at another time.

Below should be a picture of both.

S-9 and modified Colt

So just to be clear, my PGS mags fail to feed when not even in the gun so nothing to do with the lower but that doesn't rule the lower out either.
So maybe I got a bad batch?  I also tried swapping the spring in mine with an UZI and 33rnd Glock spring which helped but still wouldn't just stripping rounds off with my thumb also.   Also tried swapping just the spring and also spring and follower from the factory 30 round CZ mag so I suspect something is wrong with the bodies of my PGS mags.
If yours work then maybe I got a bad batch.  My buddy ordered 4 at the same time I did and has the same issues so maybe we both have bad mags?
+1, I didn't pay attention to the ejector either since I went straight to the CMMG RDB system.  Yeah round bent rod for the BHO but who cares if it works.
I would use the KS47 upper but I'm using my hybrid Dissent build which required me to mill out a new cam pin relief and the KS47 upper is too thin where the new relief needs to be.  Also all the Dissent uppers have ejection ports that are more narrow but taller.  Which my Midway uppers I'm using are actually pretty close to so I'm stuck with those.  The Midway uppers are actually really nice and cheap.
Funny you mentioned that about the Sterling mags.  Back in the day, I think I bought like 50 Sterling mags to mod for the 9mm M16 but never got around to it.  I sold them off and just found another one a few days ago.  
I'm doing another hybrid CMMG RDB 'PhaseII" build for one of the guys at the SMG match and he wants to run Colt mags so I need to test that again.  Haven't touched that since I did it mentioned on my site: https://c3junkie.com/?page_id=374
One thing I never bothered to do was to weld on the bottom of the feed block to make a stop to prevent over insertion of the Colt mags.
Video below is my old CMMG RDB setup using a modified 5.45 bolt with the custom fixed ejector.

Couple hundred rounds today. Only malfunctions (double feeds) were attributable to a specific mag (Gen2).
Really is a nice shooting upper.
All hand loads.

20yds, "sorta" supported, rapid fire.

lol. I'm looking to run this setup in a PCC course. Just realized I have no sling for this. Not sure how to set one up; not seeing any attachment points on the upper at all.

ETA: Actually, figured out a top rail sling attachment.

Not a BRT gun (I made mine first), but I use a Magpul RSA behind my front sight.

Thanks!

Getting the one that sticks out the side for a loop or HK clip.