First of all, I know this is totally unnecessary and I'm doing this just for fun and academic purposes.

On that note, I don't think fluted chambers are even necessary on MP5's.  One of my friends maintains the guns for a MG rental range and routinely swaps out MP5 barrels with cheap barrels he turns on his lathe and they are NOT fluted and work fine for blasting.

I have no extraction or reliability issues with my full auto hybrid Dissent build I have posted about already and for those not familiar it is all on my website here: https://c3junkie.com/?page_id=1733

IIRC, as far as MP5s go, a few years back an MG rental guy posted on a firearms website that they use (lower cost) unfluted barrels on their rental MP5s, BUT they use low power (and lower cost) ammo that doesn't swell and jam.  Cheaper barrels AND cheaper ammo - a win-win!

MHO, YMMV, etc.

I predict it will work fine

Awesome, let's meet at the range and lets drop your CMMG RDB bolt in there to test!!  ...and don't blame me for any issues like your extractor lip shearing off if it happens! lol

I'm cool with that! Although, you'll probably have tested it already by then.

This may be enough that it could run with a Scheel buffer, or even a Maxim?

I think it would work fine for those.....but just like I stated in my initial post I don't think any of this is needed even in an MP5.  
People are cheap (including me).  I wouldn't pay someone to do this.  I only did it since I have the means to do it myself.

For most SMG's and MG's, I like a cyclic rate around 600 RPM.  So in most cases, I am trying to slow the guns down but retain smoothness.  If this setup works, and the expanding brass isn't fighting the extractor, I would anticipate an increase in the cyclic rate which I do not want but again just an experiment.  The only configurations I have that I wouldn't mind making faster are an open bolt full size UZI and my FM9 belt fed upper.  
I have no idea what a fluted chamber would do in an open bolt gun.  With the belt fed FM9, it was super slow running in the high 400's but since I dropped a Maxim RDB in it, I clocked it at 602RPM so just about perfect.  

Regarding the CMMG RDB, below is a 3 second video I did a long time ago troubleshooting all the ejector spring failures:
This is with a factory Banshee barrel and bolt in there.  Every RDB I've seen has this slop.
That is what kills the ejector springs on the Banshee's which again isn't an issue with the Dissent since they now use a fixed ejector in the upper.
Anyways, I would think that the brass would expand into the flutes during this little bit of straight rearward travel.

I thought the opposite in my prediction above as the purpose of the flutes is to equalize the pressure on the outside surface of the case, thus prevent it from sticking and ‘float’ the case out of the chamber.

I agree with what you are saying and maybe I just have issues trying to articulate my concerns.
I fired my first full auto MP5 like 30 years ago and it was an MP5SD that was an issued weapon to a SWAT officer I knew.  I had been reloading since I was 14 so I always paid attention to the brass and his brass was really mangled up from the flutes.  I did a Google search and they were like this:


I wouldn't try to reload that brass.
Here is another view:


His brass probably had even deeper grooves than that.  It didn't jam at all though.  I'm sure it was some hot duty ammo he used.  It was at an indoor range and the SD really gassed me out too.

So I always had that impression in my mind of what MP5's do to brass.  Since then I've had friends with full auto HK G3's as well which really mangle the brass.  

But then I also had friends with MP5's and the brass is like I have pictured in my first post from my own MP5's and the brass while discolored isn't deformed to the point that I won't reload it.  I have never had deformed brass like that out of any guns I have with fluted chambers.

So all that said, I keep imagining worst case where the brass is getting 'fire formed' in the chamber like in the two pictures above and the bolt is trying to rotate.

That's interesting.  The brass ejected from my CMMG 9mm is completely coated black with carbon.  The only ammunition I have found that leaves normal unsooted brass is 124 grain NATO.

Wow, I haven’t seen it deformed like that! I’m used to some light carbon lines like in your OP.

I've never had that issue with my CMMG  but I primarily just shoot my reloads.  What can are you using?

So the barrel OD is so small on the MP5 that I was worried that I wouldn't have enough shoulder for the extension so I purposely left it a little long and had some light hits. So initial testing is positive but I need to pull the barrel and face the shoulder some to tighten up the headspacing for reliable operation. Then I can hopefully get some FA cyclic rate results.

Additional details on my site: https://c3junkie.com/?page_id=2096
I am going to run it like this for a while.

FWIW, I run a Stribog, which is also roller delayed so same brass ectraction conditions as an MP5.  Not only is the factory chamber not fluted, it's stepped.   Runs fine with anything I feed it.  I've never had any form of extraction or ejection failure in the 4000 rounds or so of mixed random ammo of every type.  I'm not convinced flutting does much beyond a place for extreme fouling to go, in a roller delay

I also run a P7, which is a fluted chamber as well.  I reload its brass all day long just fine.  But then, the flute marks are only a little deformation, nothing like that photo.   That brass life is different than an MP5, since it's a gas delayed blowback, and will start and have full movement under different timing and conditions than a roller delayed Mp5, and I suspect there is some.value to fluting a P7 - though I don't think Walther or Alien flute their gas delayed chambers - so maybe not.

I've got over 25K rounds through my previous CMMG RDB configuration w/ zero extraction issues either.
As I originally posted, I did this experiment for educational / entertainment purposes not to address any shortcomings of the CMMG RDB system.
Also, I have previously posted that my CMMG RDB configurations already extract on their own when suppressed with no suppressor mounted.

They typically do require an extractor to be installed to extract with no suppressor mounted however but with this fluted chamber, they will extract with no extractor installed and no suppressor mounted which I think is impressive.
It is my understanding that the purpose of a fluted chamber is for the gases to 'float' the fired round and has nothing to do with having a place for fouling to go.

That's the theory.  In a P7, which is gas-retarded blow-back, I think it actually might help. I know you can run a P7 without an extractor even, and it'll still run.  But with a roller-delayed action, like MP5 or Stribog (and likely the CMMG RDB system as well) - I'm not sure it is necessary or does anything.  The Stribog doesn't need it, and they even went with a stepped chamber, which can increase extraction requirements, and it runs fine.

Yes, as I've stated several times already NOT needed for MP5 or CMMG RDB.  My point is that it does work since it does 'float' the fired case and runs without the extractor installed and no suppressor mounted when it wouldn't do that with a non-fluted chamber.

I also run about 99% full auto and suppressed with my CMMG RDB builds and have never broken an extractor in the past +25k rounds.