Made this one topic so I'm not spamming the forum.

What exactly is different about 9MM(or "pistol?") uppers compared to .223/5.56(or "rifle?") uppers? Are the BCGs drastically different that the upper needs to be specialized in a typical 9MM system?

Secondly, watched an interesting video by Forgotten Weapons on Youtube that talked about the Radial Delayed Blowback system from CMMG. From the looks of it, you could use a standard rifle buffer/spring combo so it would only really need an upper change.

The person also pointed out that the BCG for the RDB system was almost identical to a standard BCG, just some slight machining differences.

If anyone here has experience with the CMMG RDB system, I'd be interested to hear about it. I know a BCG/barrel combo is available, and I'm curious if it would need much more. I think the only real difference was the CMMG system needed an enlarged ejection port, but if that's something I could do with my favorite upper (I'm thinking side charger or maybe dual,) it would be a nice system to have.

I'd be using the system with their adapted magazines so my lower would take standard AR type mags if I decided to build it.

First of all you have straight blowback and then their is the CMMG RDB.

For straight blowback, there is no carrier just a bolt that goes back and forth which is typically weighted.  
Typically you also run a heavier and longer buffer as well as stronger buffer spring to help mitigate the recoil.  

Additionally most aftermarket triggers are not warrantied to run in both 556 and 9mm straight blow back actions due to the recoil impulse.

Most straight blowback designs are of the Colt pattern but there is also the Olympic pattern that uses a spring loaded ejector in the upper receiver itself.  

You can use a standard AR upper in the blowback setups and just not use the relief for the cam pin.  The ejection port is wider than it needs to be and typically most bolts are not notched for the forward assist.

You are correct on your descriptions of the CMMG RDB.  
CMMG has several videos on Youtube.  Here is one: https://www.youtube.com/watch?v=eNiCyfp5nXA&t=23s

The CMMG RDB was initially introduced as the 'Guard'.  
Here is a long thread on them:https://www.ar15.com/forums/ar-15/Thoughts-on-the-CMMG-Guard-/15-713060/
That was back when CMMG was more active on the forums and would chime in.
There are a lot of threads on here about them.  A lot of people also refer to them as the CMMG Banshee.  The Banshee marketing term is confusing as it is basically anything with a short barrel.  So we could be talking 22LR.  Also more confusing is that CMMG still sells their Colt pattern straight blowback 9mm and not just the RDB.

I have the CMMG RDB in 9mm / .40SW and .45ACP.  I run them all in full auto. I typically just get their BCG/BBL combos.  I have had issues with the spring loaded ejector in 9mm and .40SW.   However, there are some saying they are still on their original spring after over 3K rounds.  Myself and some friends have had them die at around 1K rounds.

Here is my website with all my details on tuning and troubleshooting the ejector: http://www.c3junkie.com/?page_id=164

I no longer run any of my straight blowback 9mm setups.  Much smoother and the ability to swap uppers with no changes in the lower is great (when talking 9mm and using the Endomags).....BTW, Mean Arms makes the 'ARC' mag inserts for CMMG.

I put a CMMG RDB system together for USPSA PCC competition... I used a New Frontier Glock format billet lower and a standard Aero Precision upper... the RDB system has the locking lugs angled to a point to facilitate unlocking with the recoil impulse, and the sharp teeth may cause a problem if they drag on the next presenting cartridge or on the magazine lip when feeding or extracting ... magazine height is critical... ... I spent quite a bit of time trying to get magazine lip clearance and shelf height on the catch correct... Magpul magazines sit lower, and worked without any problems for me, but at the time only held 29 rounds... my ETS 40 round mags occasionally get a groove cut in the lip... no experience with the non competition magazines

There also isn't a hole in the upper for the gas to to go through on a dedicated blowback 9mm upper since it isn't needed.

If you were ot go buy a stripped "22lr/9mm upper" upper from RTB or wherever, the ejection port door will be a little shorter I believe.  Slightly different shape to the deflector bump.  No forward assist.  There may or may not be a hole drilled where the gas tube would normally go.  Haven't laid hands on one, so there may or may not be other internal differences (see below about the FM upper and the clearance for the bolt cam pin).

If you were to buy a complete 9mm upper from FM with the forward non-reciprocating side charging, there wouldn't be a spot for a charging handle to latch into at the back.  And there wouldn't be some internal machining that the bolt cam pin would need for clearance ('cause there ain't one on a 9mm or 22lr bolt...).  So a standard 556 BCG only goes in about half way before stopping.  Gas tube hole has the rod for the charging handle run through it, don't have a gas tube handy to compare diameter but it looks a hair smaller than a gas tube.  

If you were to go buy a stripped slick-side upper from Anderson (no forward assist, no dust cover) you could build 9mm, 223, 22lr, whatever you want on it.   Port opening would be bigger than needed technically, but that has never bothered me.

If you were to go buy a regular stripped upper from wherever with a dust cover and forward assist, the dust cover may or may not cause issues or need replacing, and the forward assist would just kinda be there since there are typically no cuts in a 9mm bolt to let it engage to push forward.  Those two things aside, you could build a 9mm or 22lr on them no problem.

Sounds like a fairly glowing recommendation, even considering the flaws.

What kind of items would you recommend for a Whoops or Chit kit? Multiple ejector springs? Maybe just have a complete bolt?

I'm looking to build this into a pistol kit for a center console gun, with a quick release barrel setup. Additionally, I could swap out to a rifle length barrel, and slap it on a rifle lower for cheap plinking.

I'm not going for a pistol brace, just a shorter buffer tube and spring, and plan on just shooting it like a pistol. Will probably use a magwell grip, since that's going to give the most practical ergonomics for doing that.

From what I've read, it does need an enlarged ejection port, and I'm considering a dual charging receiver so it would have to be opened up a bit. I'd also need the bolt modified. Since there are about four gunsmiths local to me, I don't see that as any kind of obstacle.

Thank you for the advice. I'm on my way to getting this thing built.

Adding a magwell grip, or anything forward of the primary pistol grip that could even *imply* that a second hand is intended to be used while firing (angled fore grip, hand stop, etc.) could allow ATF to classify it as an AOW or SBR instead of a pistol if asked by a prosecutor, based on the FOIA releases we have seen from them recently.  Not trying to scare anyone, but forewarned is fore-armed.

Anyone else willing to bet that ATF use two hands on their pistols?

It needs a full length ejection port as it uses an ejector in the bolt similar to a 5.56 gun, but not necessarily enlarged in height like a 450 BM port.

CMMG clearly states their 9mm/.45/.40/10mm radial kits "requires enlarged ejection port for reliable operation".

Otherwise, it can be a completely normal AR15 upper.

Aero makes a high quality enlarged-port-door upper that takes standard upper parts and looks like a standard upper for all intents and purposes.
https://www.aeroprecisionusa.com/ar15-xl-assembled-upper-receiver-anodized-black

Many have used standard 5.56 port uppers for their 9mm builds.

OK. And?  CMMG still recommends them for the most reliable operation.
https://cmmginc.com/product/barrel-and-bcg-kit-8-4140cm-sbn-9mm/

Lots of people do things that aren't recommended all the time... it's thursday 11am and I'm supposed to be working, yet here I am

CMMG does sell the MK10 ejector spring separately so I'd recommend getting a few of those.

Ugh.  Let's not totally hijack the thread.  The laws the ATF is charged with interpreting specifically defines a pistol as "designed and intended" to be fired with one hand.  Current pistol shooting training and doctrine is irrelevant, especially to a power-hungry bureaucracy.

And???

If it works, it works.

And, I'd wager you also correct people's you'res and yours and threws and throughs on fb and use a torque wrench on castle nuts too

OP said he wasn't going to install a brace so.....

re: enlarged ejection port & extraction problems.. I shot my CMMG RDB in competition for around 1500 rounds before I encountered the extractor problem presented as other have mentioned , followed with the recommendations to use the 10mm parts.... up to that time the barrel was in a stock Precision Aero upper with 5.56 ejection port size...bolt travel was limited to about 3/8" past the bolt hold open (which is not used on my competition carbine).... my extractor problem was not with the ejector spring as others have commented, but that the polymer plug insert within the extractor spring had hardened... at that time I replaced the extractor spring with a heavy duty one from BMC without a plug or O ring (Memphis Mechanic a forum member on the Brian Enos site recommended as he had the same problem), and only then stuck the upper in the milling machine and slightly opened up the ejection port... due to the covid experience, have not shot much more than 300 rounds since then, as I've been focusing on Steel Challenge and not USPSA matches... regards

Hmmm.

I'm looking at something like this: https://www.dhresource.com/0x0s/f2-albu-g2-M01-CD-93-rBVaGlaXyvWAR4nNAALKs1Q2iFA144.jpg/fab-mwg-defense-magazine-well-gun-grip-and.jpg

However, considering how wishy washy ATF can be, I'll probably just have to Google their latest opinion right before I walk out the door to go to the range.

Thanks for the heads up.

Perfect. I'll add that to the list.

I'll mention one other bolt feature to monitor.  I've started to get more diligent about bolt cleanliness ; after temporarily switching to WSF powder I had noticed a lot of build-up on the bolt.  Specifically, I was concerned about build-up in the ejector channel. This last cleaning I noted when reinstalling the bolt that the extractor was creating drag at the bolt/carrier interface.  Turns out the pin holes in the extractor had ovalized causing the bad end of the extractor to stand proud of the bolt circumference.  I have not made a practice of oiling the extractor pin, but I'm going to start doing so.  I'm not sure the drag would cause FTE, but when the hole is enlarged the extractor claw engagement decreases and that can't be a good thing.  The symptom was a cascade of FTE which is not what I have experienced when the ejector spring has lost tension.  I noted in the main AR15 section of this forum the recommendation to lube the extractor pin.

dedicated 9mm AR-15 (AR-9) upper won't have the usual AR-15 brass deflector bump, since they often use the 9mm-specific gas deflector instead.

and the no-gas-tube-hole.

I just picked up a Cmmg RDB kit built on a vltor pcc upper: https://www.vltor.com/shop/ar/upper-receivers/vltor-pcc-mur-pistol-caliber-upper-receiver/

Do you think the ejection port is big enough?

It isn't long enough.  You should use an upper that has a standard length opening for 5.56 not a straight blowback PCC orientated one.

This is a link to an old thread that CMMG posted on several times: https://www.ar15.com/forums/AR-15/-ARCHIVED-THREAD-Thoughts-on-the-CMMG-Guard-/15-713060/?page=6
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>.
"We have not included a gas deflector on our Guard rifles so far. Current calibers offered do not need the deflector. Gases are not as much of an issue with RDB compared to blowback.

We did test the rifles with the deflector. Current blowback style deflector with RDB would periodically bounce empty case back into action. Didnt happen often, but it did happen.


I suspect a small variation in gas deflector angle would take care of this. We weren't worried about testing it more  since RDB got rid of much of the gasses."
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

Since the RDB uses the spring loaded ejector and we are dealing with the shorter 9mm case, it ejects the case at erratic angles.
Look at some youtube videos and watch the ejection angles.  Like this: https://www.youtube.com/watch?v=VqR4Oafsa1U

I figured. Bought it off another member here because I wanted the RDB kit without waiting 6 months. New upper is much easier to find. the Vltor does have a removeable blast shield so I may try it without as a single shot to see if it ejects reliably. If not, 223/5.56 upper and a spare pcc. Thanks @Amphibian

I bought the  RDB barrel and bcg from Cmmg in 45 acp

It’s a fairly unique concept and kudos for them developing it.

The BGC and bolt although superficially similar to the standard AR will not interchange the bolt carrier is bored deeper and to a larger diameter.

The bolt is also bigger. The cam pin is round on top with 1/2 of it cut lower

The fp, ejector, extractor, bolt retaining key are all the same

The Colt and Olympic systems are blowback.
The CMMG system uses a locked bolt the opens in a delayed fashion that is a hybrid.

Well I got around to finishing and test firing my radiall delayed pcc build and it was mostly a success. Ejection wasn't an issue save one stovepipe. Did have some problems loading the second round specific on a few mags and some failures if the lrbho. Swapped the carbine buffer for an H2 and I'll see if that makes a difference.

Is that a standard AR upper with the ejection port modified?

That may have been caused by that deflector being on there...bear in mind that as the ejector spring gets weaker, the ejection pattern will get more erratic and you may get more stovepipes and ejection failures.

The Achille's heal of the CMMG RDB system is the spring loaded ejector....personally I would do everything possible to make sure that there is nothing to inhibit that.

Agree.  The RDB doesn't have a solid ejector and it may take a little longer for the case to leave the port.  I would try it without that and see if it runs better.

Swapped the standard buffer for an H2 and it ran flawlessly. Ejects right around 230-3 oclock, feeds flawlessly, no more stovepipes, and it eats brass/steel/aluminum new and reman! Right now it's more work than it's worth to remove the deflector but I will if I see any more issues with brass coming back into the chamber.

Slowing down the bolt likely gives the ejector more time to clear the case before getting to the deflector.  More than one way to skin that cat.  Glad you got it running well.'

So I see that CMMG is now offering Colt-pattern RDB guns, in addition to their Glock pattern, P320 pattern, and Endomag pattern 9mm designs.

Are there advantages to the Colt-pattern vice the Glock pattern? I've got a couple of spare 24- and 33-round Glock mags, and was originally planning on going in that direction for a 16" PCC build, but now I'm torn.

Single-feed Glock or Double-feed Colt? What does the hive mind prefer?

Aesthetics, at least for me.  Glock mag angle looks goofy on an AR.

Well, I guess I'm sold on the Colt-pattern RDB. As much as I'm sure it's just all subtle differences, I think I'm just more down for double-stack, double-feed than I am double-stack, single feed.

CMMG is marketing a conversion kit for their old blowback Colt-pattern lowers the "Mk 9", and it replaces the buffer and action spring, the mag release, the bolt stop, and the feed ramp. The new parts are all optimized for the RDB.

I saw that the old CMMG Mk 9 lowers were in stock and cheap, so I've got one headed my way, along with that conversion kit. I expect that this build will take a while to complete, mainly because parts are slow to find and acquire, but I'm looking forward to living the PCC life.

Several of us did Colt mag conversions for the CMMG right after it came out...myself included.
http://www.c3junkie.com/?page_id=374


I did that before the Endomags came out.
I now just run Endomags as I like using the same mag pouches I use for 556 and identical load out.

Agreed.  Add to that, they feed everything and anything.

I remember ya’ll making that happen, and being very impressed.

I like CMMG did the engineering/gunsmithing on their end to change the bolt stop, mag release, and feed ramp with their lower. I definitely get that there are a lot of ways to end up with a reliable gun, I just lack those sort of skills myself, and enjoy factory solutions.