{"id":1733,"date":"2024-02-03T22:31:25","date_gmt":"2024-02-03T22:31:25","guid":{"rendered":"https:\/\/c3junkie.com\/?page_id=1733"},"modified":"2025-10-24T00:33:46","modified_gmt":"2025-10-24T00:33:46","slug":"ultimate-full-auto-cmmg-rdb-fixed-ejector-project","status":"publish","type":"page","link":"https:\/\/c3junkie.com\/?page_id=1733","title":{"rendered":"Ultimate Full Auto CMMG RDB Fixed Ejector Project (hybrid Dissent 9mm)"},"content":{"rendered":"
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The goal is to have a smooth<\/em>, reliable<\/em><\/span> and tune-able<\/em><\/span> delayed blowback full auto 9mm configuration for use with a RR \/ RDIAS or RLL utilizing the CMMG RDB (Radially Delayed Blowback) system.\u00a0 I presume it should also work with binary triggers but I don’t have any experience with those.\u00a0 Or if you just want to tune a 9mm fixed ejector Dissent. \u00a0 If you are reading this, you probably already know that the original CMMG ‘Guard’ (now referred to as the Banshee which is confusing since the Banshee term is also used for non-RDB configurations) has issues where the ejector spring fails.\u00a0 For me and many others, it seems to be around the 1000 round mark but I have seen people post as little as 300 rounds they have failures as well.\u00a0 See this link if you want more detail on the spring loaded ejector issues: CMMG RDB Ejector Spring Issues<\/a>.\u00a0 The CMMG Dissent was released in 9mm and they finally went with a fixed ejector which is great but it is in a proprietary side charging upper that lost the ability to trip a sear and dropped the buffer tube so no more tuning with springs and buffers.\u00a0 Which BTW, the stock<\/strong><\/span> 9mm CMMG Dissent using their buffer-less setup with the internal recoil springs is NOT<\/span><\/strong> as smooth as an MP5 which isn’t surprising.\u00a0 So I made what I will call a ‘CE – carrier extension’ to trip the sear and use the buffer tube again.\u00a0 I initially made two of these, one from 7075 aluminum and one from stainless steel as I didn’t know if the aluminum would break from the shock (however, I believe that using a hydraulic buffer should aid in absorbing the shock based on all the high speed video I have done in the past).<\/p>\n

As I have mentioned many times I was never impressed with the recoil impulse of an out of the box CMMG RDB.\u00a0 Is it better than a typical straight blowback AR9?\u00a0 Yes.\u00a0 Is it as smooth as an MP5 – no way.\u00a0 Let’s say it is like 60%<\/strong><\/span> there out of the box.\u00a0 Then using my combination documented here: My OLD 9mm CMMG RDB Setup<\/a> which initially consisted of using a Tubb flat spring, A5 buffer tube and the Kynshot hydraulic RB5007 I would say it got it to about 90%<\/span><\/strong> of the MP5 but it also has a nicer cyclic rate than an MP5 with that configuration.\u00a0 Going to the .40SW<\/span><\/strong>bolt (60 degree vs 50 degree<\/span>)<\/strong> for use in 9mm is what brought it to a point that matched the MP5 (in my opinion and several others that have shot my setup).\u00a0 However, the recoil impulse is so soft with the .40SW bolt, it isn’t moving fast enough for the spring loaded ejector to kick the brass out so a fixed ejector was required but then slotting the RDB bolt for the fixed ejector goes right across where the cam pin is and the bolt will eventually crack like mine did.\u00a0 So it all worked great but you do need to keep an eye on the health of the bolt.<\/p>\n

Phase I<\/strong><\/span> – Make a custom ‘carrier extension’ to allow the\u00a0 9mm CMMG Dissent upper to trip the auto sear and utilize the buffer tube to leverage all the aftermarket springs \/ buffers.
\nPros<\/span><\/em>: This requires the least amount of machine work.
\nCons<\/span><\/em>: The Dissent BCG is heavier than the Phase II carrier so you are starting off with more reciprocating mass.
\nUpper is proprietary so may be fine if you like their side charging setup.
\nUpper is also heaver.
\nIt is a NO-Go if you want to do an integrally suppressed build.
\nTuning operating window is smaller due to the starting off with more reciprocating mass.
\nPhase II<\/strong><\/span> – Buy the 9mm CMMG Dissent BCG group which is currently available separately and retrofit \/ build out a new upper using a standard CH and existing CMMG ‘Guard \/ Banshee’ barrels or make my own RDB barrels<\/a>.
\nPros<\/span><\/em>: Less reciprocating mass (at the time of me writing this, I think it is smoother than what is possible with Phase I)
\nLarger operating window for tuning
\nCompatible with other standard charging handles
\nSeveral upper receiver options (just need to be thick enough as detailed on this page)
\nCompatible with other side charging options like: https:\/\/typezerodesign.com\/<\/a> or https:\/\/www.gibbzarms.com\/<\/a> (not tested – may conflict with fixed ejector)
\n<\/a>\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Compatible with making an integrally suppressed (SD) build
\nCons<\/span><\/em>: A lot more machine work<\/p>\n

Note that with both Phase I and Phase II, I am making permanent modifications to some parts so forget about expecting any kind of support from CMMG or any other vendors mentioned or anyone including myself.\u00a0 If any of you want to copy me, it is at your own risk.<\/p>\n

I have built over a dozen for friends and we are all running them 100% in full auto.\u00a0 I also make them for anyone interested.\u00a0 Contact Me<\/a> if interested.<\/p>\n

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Picture below shows what the bottom of the custom aluminum ‘carrier extension’ looks like.\u00a0 A steel sear trip insert is secured by a bolt that also doubles to prevent the wave spring from going solid height.\u00a0 I bore out the carrier extension to use the same weights used in cheap AR carbine buffers.\u00a0 I prefer using the .6oz steel weight.\u00a0 The 1.4oz Tungsten weight feels too bouncy to me.\u00a0 Note that I typically run full auto suppressed with a short NON-ported barrel.\u00a0 If you run a long barrel, maybe the Tungsten is better.\u00a0 Likewise, as mentioned way down on this page is a BRT ported barrel with a B&T suppressor.\u00a0 Setup is super smooth as he wasn’t consistently getting LRBHO but he was also running 147Gr with an integral.\u00a0 He switched to 124Gr and also removed the steel weight (and spring, set screw) to re-gain LRBHO but still super smooth.
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Now that CMMG has released a FE (Fixed Ejector) Banshee, people have asked me if it is still worth it to get a PhaseII.<\/p>\n

Below are the main differences.<\/p>\n

With the PhaseII, you can remove the rear lug so that you can have an ‘extended stroke’ like I did in 556 documented here: Maximizing BCG Stroke length &#8211; &#8220;M16 Ultimax&#8221;<\/a> Also documented by one of my customers running in semi only here: https:\/\/www.reddit.com\/r\/AR9\/comments\/1ik1ceo\/the_phaseii_custom_radial_delayed_blowback_by\/<\/a><\/p>\n

The design of my aluminum ‘CE’ allows the use of live mass inside the CE vs dead weight.\u00a0 Like a real MP5 carrier has tungsten powder inside of it to provide more of a dead blow hammer affect.<\/p>\n

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