{"id":221,"date":"2020-04-04T01:57:10","date_gmt":"2020-04-04T01:57:10","guid":{"rendered":"https:\/\/c3junkie.com\/?page_id=221"},"modified":"2025-04-20T22:41:14","modified_gmt":"2025-04-20T22:41:14","slug":"cmmg-rdb-ejector-spring-issues","status":"publish","type":"page","link":"https:\/\/c3junkie.com\/?page_id=221","title":{"rendered":"CMMG RDB Ejector Spring Issues"},"content":{"rendered":"
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TLDR Version:<\/p>\n

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  1. The slop between the bolt lugs and the extension lugs kills the ejector springs.\u00a0 Scroll down to look at the videos for clarification.\u00a0 The ejector spring in a 556 is not an issue since that bolt is truly locked.\u00a0 Not so with a delayed blowback setup.\u00a0 I am not a fan of any ejector spring that is inline with the recoil impulse of a delayed action.\u00a0 I had the same failures with the MEAN Arms bearing delay.\u00a0 It actually lasted less rounds than the CMMG RDB ‘Guard\/Banshee’.<\/li>\n
  2. If you remove the slop by getting a custom barrel made, the impulse will be so soft that the case won’t eject and then a custom fixed ejector is required but then that will weaken the bolt which will eventually crack (mine did at around 25K rounds).<\/li>\n
  3. I have never personally seen a CMMG RDB ‘Guard\/Banshee’ run hard (full auto, suppressed) for over 1500 rounds without the ejector spring failing.\u00a0 I had<\/strong><\/span>\u00a0 (2) 9mm CMMG RDB ‘Guard\/Banshee’ setups and they both had the issues.\u00a0 I have several friends that all suffered the same fate.\u00a0 I see online some people saying they have more than 1500 rounds with no issues.\u00a0 If this is true, my only guess is their bolt \/ extension slop happens to be ‘perfect’, I just have never seen it myself.<\/li>\n
  4. Some people report of immediate failures not even making it through the first 50 round box of ammo.\u00a0 In those cases, I would say to make sure you are running the CMMG recommended baseline configuration: milspec spring, standard carbine spring, standard carbine length buffer tube, their upper or a .458 \/ XL enlarged upper and quality factory ammo.\u00a0 I know many have posted great success with using a standard 556 upper which is fine if you don’t have problems.\u00a0 I’m saying all this to minimize the variables and get factory support.\u00a0 They will probably tell you to go to a heavier buffer, send you a tuning kit and maybe a replacement bolt depending on age.<\/li>\n
  5. Are you running suppressed?\u00a0 See my video below where I had an early barrel which had excessive slop between the bolt and extension lugs which was resulting in broken cam pins, broken firing pins, breaking the extractors and killing the FP springs quickly.\u00a0 I noticed that if I removed the extractor from the bolt<\/strong>, that barrel would extract rounds with NO suppressor mounted while the ‘good’ CMMG 9mm and .40SW barrels would only extract the round if a suppressor was mounted.\u00a0 It was that video that I finally got CMMG to replace my original barrel as proof that there was something unique going on with that barrel.<\/li>\n
  6. I run about 99.9% suppressed and as mentioned above, it extracts just fine with a suppressor mounted even with no extractor installed in the bolt.\u00a0 I of course do use an extractor since you still need it to clear a live round.\u00a0 However, IME, it doesn’t have to work hard.\u00a0 I know many recommend getting the BCM extractor kit but I never found it helping the ejection failure issues myself.<\/li>\n
  7. IME, best cased scenario is that you get around 1K before the MK10 ejector spring fails.\u00a0 Which BTW, if you read the entire article, you will see I have tested a bunch of different ejector springs, including the Bexar Inconel spring and nothing is as strong as the CMMG MK10 spring. \u00a0 Don’t waste your time measuring the MK10 spring length as it is shorter but has thicker gauge wire coils.\u00a0 It is best that you measure the strength of the ejector spring and get a feel for when it is time to replace.\u00a0 It is also faster since you are not taking the spring out of the bolt to do this.\u00a0 If you are at 5lbs or less, it will start to fail you.\u00a0 A new MK10 spring is around 10lbs to compress.<\/li>\n
  8. Have a spare bolt with new spring ready to go.<\/li>\n
  9. All the above is my advice if you want to stick with the system.\u00a0 Many will say this is too much of a PITA.\u00a0 However when you consider that the bolt \/ BCG combo is under $400 to get a delayed action that can be very smooth (especially if you use my A5 \/ RB5007 and flat spring combo) I think it is still worth it.\u00a0 Especially if you are looking for this in the AR15 \/ M16 platform and maintain the same manual of arms.\u00a0 Bonus points for SBR, RLL, RR, RDIAS owners.<\/li>\n
  10. All that said, I dumped my 9mm CMMG RDB ‘Guard\/Banshee’ for my hybrid CMMG RDB Dissent which uses a fixed ejector and tune-able as documented here: https:\/\/c3junkie.com\/?page_id=1733<\/a>.
    \nI will also say that I have never impressed with the recoil impulse of a STOCK CMMG ‘Guard\/Banshee’ OR the new 9mm CMMG Dissent.\u00a0 However if tuned they both can be amazingly smooth.
    \nI still have my .40SW CMMG RDB ‘Guard\/Banshee’ and just deal with swapping ejector springs at around the 800 round mark with that caliber.\u00a0 I also have it in .45ACP which I don’t shoot as much and haven’t seen ejector spring failure posts in .45 which also operates at a lower pressure.
    \nRegarding tuning, unless you do custom machine work like I did for the Dissent, it cannot be tuned.\u00a0 The 9mm Dissent does have some bolt bounce that you can’t easily do anything about without custom work.\u00a0 The .45ACP ‘Guard\/Banshee’ could handle .45 Super but the new Dissent cannot since it cannot be tuned.<\/li>\n<\/ol>\n

    The CMMG RDB system was initially introduced in .45ACP and they came out with a proprietary ejector spring for it.\u00a0 It is shorter than a standard 556 ejector spring and there have been people thinking it was weaker but it is thicker and actually stronger than a typical 556 ejector spring when new.<\/p>\n

    I was never interested in the .45ACP CMMG so waited till the 9mm version came out.\u00a0 \u00a0However, I do have friends with the .45ACP RDB’s and none of them have had ejector spring failures but I don’t think any of them have run them as much as 9mm.\u00a0 Nor have I seen posts online about the 45 wearing out ejection springs.\u00a0 So I don’t know if .45ACP have the same problem.<\/p>\n

    When the 9mm came out they used standard 556 ejector springs and some people started reported ejection failures.\u00a0 CMMG then started to supply the 9mm RDB bolts with the same spring they were using in the 45ACP RDB.<\/p>\n

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    In 9mm I was seeing failures around 1000 rounds.\u00a0 I also have the CMMG RDB in .40SW and the ejector spring failed around 900 rounds in that caliber as well.\u00a0 CMMG then released the MK10 (10mm RDB) in late 2019.\u00a0 They switched to a new material for the ejector spring that is being used in the MK10 and have been offering this new spring for those encountering ejector spring failures in their 9mm’s, .40SW and I guess .45ACP’s (although as I mentioned previously I haven’t heard of it happening).<\/p>\n

    In the picture below you can see that I also tried an HK416 spring and although it is a lot longer it isn’t that stiff and ended up not lasting long either.\"\"<\/p>\n

    Picture below shows how strong a new “45ACP” Guard spring is in a 9mm RDB bolt.\"\"<\/p>\n

    Picture below shows a new Wilson Combat 556 spring.\u00a0 This is barely enough to be reliable.\u00a0 Note that I see about the same amount of force for other ejector springs, Sprinco, Tubb, Sig and no-name brands, etc…only the CMMG RDB springs are significantly stronger than other 556 ejector springs.
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    In my testing when it gets around 5lbs or less, you are going to start having ejection failures.
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    Brand new HK 416 bolt only has a little over 6lbs of force.
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    After only 120 rounds, you can see how much shorter the HK416 spring is.\"\"<\/p>\n

    Spring strength is close to 5 lbs (failure point).
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    Pictured below is a DPMS GII ejector, notice how the ejector pin has a built in guide to help prevent spring deformation.\"\"<\/p>\n

    I tried to replicate this by using a cut nail
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    I did some limited testing with this but no clear indication of increasing longevity as I stopped when I saw that CMMG mentioned that the new (as of late 2019) MK10 spring should resolve these issues.\u00a0 The picture below is using a CMMG .45 ejector spring not the MK10 spring.
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    MK10 spring length and strength when new.
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    I am running the MK10 spring in my .40SW RDB setup and at about 700 rounds so far with no issues but still haven’t gone past 1000 rounds yet to confirm if it lasts longer than the previous 45 RDB ejector spring.***5\/3\/20 UPDATE***\u00a0 Ran close to 600 rounds today at a SMG match.\u00a0 On the first two stages, it was flawless.\u00a0 Next stage I had 3 FTE’s.\u00a0 I thought it may have had something to do with some new reloads and so went back to the same loads I ran in the first two stages and then I had at least 6 or 7 FTE’s.\u00a0 I’m done with the spring loaded ejector at this point.<\/p>\n

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    So you may be wondering why we don’t have this problem in other AR15\/M16 calibers using the ejector in the bolt like 5.56, 7.62×39 etc…The bolt is really locked into the extension and requires the pressure to build up in the carrier to push the carrier away from the bolt while the CMMG RDB doesn’t rely on the gas.<\/p>\n

    Also, there have been several posts where some people are beyond 3 or 4K of rounds even full auto suppressed on their original ejector springs and have no ejection issues at all.<\/p>\n

    I personally believe that the root cause of the ejection spring failures is there is too much slop in the headspacing of the barrel.<\/p>\n

    This video illustrates this slop:<\/p>\n