CMMG failure to eject

[Pluder]

Hey guys, 

 

I have a delayed radial blowback CMMG which continually has an issue ejecting case. Apparently CMMG is incapable of fixing it as I have sent it in several times for this issue but it still occurs after they "repair" it. At this point I'm about to give up on it and swap out the barrel and bolt to a direct blowback. Have any of you had a similar issue? or have you swapped out a delayed radial system to direct? anything advice would be appreciated.

 

 

 

[RickT]

What ammunition are you running?  If reloads, what PF?  I've got two SBR lowers (AR with Endomags) with 8" CMMG uppers.  Using the MK10 ejection sprint and a standard AR extractor spring they run 100% as long as I'm not running soft reloads.  Even for steel challenge there is no point in soft reloads as the gun shoots much flatter with ammo with at least as a bit of zip.  This as two benefits.  First, the aforementioned flatter shooting.  Second, if you are shooting 9mm reloads you probably reuse the cases many times.  Variation in power load, neck tension caused by mixed brass and the occasional cracked case can produce an FTE, while increasing the powder charge eliminates two of these potential issues.

[Pluder]

Initially I was shooting reloads, though I was not making them extremely light. The first time I sent it back to CMMG they seemed to think that was the issue as they could not recreate a failure. They and told me they recommend Winchester white box. I gave that a try but intermittently still had issues and sent it back again. This time they replaced several springs and something else (it slips my mind at this moment as it was awhile ago). While I was waiting for it I purchased a JP and have been primarily shooting it, leaving the CMMG in the safe. However, now a friend wanted to shoot PCC, so I took it to the range to make sure it was sighted in and had an failure again (though it was only one time), but given the history, once was enough. I was using Syntech 130gr when I occurred this time. 

[Bwillis]

https://bravocompanyusa.com/bcm-extractor-spring-upgrade-kit/

[Les Snyder]

my CMMG RDB system started causing problems at about the 1000 round mark... many claim that the  ejector spring designed for the SW 40cal/10mm rifle is the solution... I had a different problem... the polymer insert inside the extractor spring had hardened to the point it would not compress... a posting by Memphis Mechanic led me to replace the extractor spring with a heavy duty one from Bravo Company (without the insert or O ring)... unfortunately the Wuhan virus has interrupted my PCC shooting for many months and was able to put only 300 or so rounds on it

Edited November 1, 2020 by Les Snyder
added comments

[Pluder]

I'll have to look into the 40 ejector spring. I don't think it's the extractor since it never fail to extract the case. Just seems like the ejector doesn't have the force to kick the case out. Maybe a little overkill is in order and they will both get replaced. 

[RickT]

39 minutes ago, Pluder said:

I'll have to look into the 40 ejector spring. I don't think it's the extractor since it never fail to extract the case. Just seems like the ejector doesn't have the force to kick the case out. Maybe a little overkill is in order and they will both get replaced. 

If the gun has been back to CMMG (how recently?) I'd be surprised that the bolt doesn't use the MK10 ejector spring. Both my uppers run with standard extractor springs, but others have had success with the extra power extractor spring.  You definitely need the upgraded ejector spring.

 

I don't suppose this can somehow be related to the magazines you're using?  I'd think CMMG tests with Glock mags.

 

A final observation: filling Endomags to capacity in combination with softer reloads can see an FTE on the first round probably due to the pressure of the next round on the bolt slowing things down a bit.  Don't know about Glock mags although I do have a Glock-configured early Guard rifle.

[Pluder]

The gun was back at CMMG a few years ago. Is there any way to know which spring is in it? by weight or color code?

 

I doubt it is a magazine issue. I use Glock mags. 

[MoRivera]

On 10/31/2020 at 8:39 PM, RickT said:

What ammunition are you running?  If reloads, what PF?  I've got two SBR lowers (AR with Endomags) with 8" CMMG uppers.  Using the MK10 ejection sprint and a standard AR extractor spring they run 100% as long as I'm not running soft reloads.  Even for steel challenge there is no point in soft reloads as the gun shoots much flatter with ammo with at least as a bit of zip.  This as two benefits.  First, the aforementioned flatter shooting.  Second, if you are shooting 9mm reloads you probably reuse the cases many times.  Variation in power load, neck tension caused by mixed brass and the occasional cracked case can produce an FTE, while increasing the powder charge eliminates two of these potential issues.

Same with me when it comes to the ejector and extractor pins, made my RDB completely reliable with two different barrel lengths and buffer weights.  But I've also only shot factory stuff and reloads that are a bit hot.  I really like this thing just wish the Endo-mag/Pmags had an extended version.

Edited November 4, 2020 by MoRivera

[RickT]

On 11/2/2020 at 5:51 PM, Pluder said:

The gun was back at CMMG a few years ago. Is there any way to know which spring is in it? by weight or color code?

 

I doubt it is a magazine issue. I use Glock mags. 

I'm almost positive the MK10 ejector spring was introduced more recently.  Just this last few weeks one of my CMMG SBRs went from 100% cycling to frequent FTE just from one shooting session to the next. Took out the ejector spring and it had shortend approximately 10% relative to a new Mk10 spring.  Changed out the spring, tested and the gun ran 100%.  I'm going to keep much better track of the number of rounds I'm putting through these guns.  Some have suggested that 1000 rounds is about the maximum out of an ejector spring.

[MoRivera]

I'll keep an eye on mine too.  Man that thing really takes a beating, right?

[RickT]

Here's the good news.  I wanted to buy some spare ejector pins.  Found a vendor, Bexar, that was selling for $0.25 each.  Bought ten and I think the shipping was $3.  Who on the interweb sells anything for 25 cents?  I do have a jig for R&R of the ejector; looking like a pretty good spend. (I could have gone to McMaster or similar, but not worth the trouble.

[bwikel]

There are a few common issues with the CMMG RDB that could be causing your issues of extraction/ejection.

• Your extractor spring is weak -  I only run extra power extractor springs (I change them annually or every 5k, whichever comes first), they are one coil longer and you don't need all the hokie o-rings to make them work.
• Your extractor may be damaged or full or carbon. When you clean your bolt take a metal pick to the extractor to assure there is no carbon build up on the hook. 
• You ejector spring is weak or dirty. If you have never pulled the ejector it is likely full of carbon and causing the spring to bind or lock up. I also run extra power ejector springs and ultrasonic clean the stripped bolt or pull the spring to remove all the carbon. 
• If you over lube your bolt it will just get dirty faster. Only run oil on the contact points where the bolt shows wear. 

By the looks of your pics the bolt is losing the empty case before the ejector has a chance to kick it out. I would start by replacing the extractor spring and detail cleaning the extractor. 

[RickT]

On 1/11/2021 at 11:08 AM, bwikel said:

There are a few common issues with the CMMG RDB that could be causing your issues of extraction/ejection.

• Your extractor spring is weak -  I only run extra power extractor springs (I change them annually or every 5k, whichever comes first), they are one coil longer and you don't need all the hokie o-rings to make them work.
• Your extractor may be damaged or full or carbon. When you clean your bolt take a metal pick to the extractor to assure there is no carbon build up on the hook. 
• You ejector spring is weak or dirty. If you have never pulled the ejector it is likely full of carbon and causing the spring to bind or lock up. I also run extra power ejector springs and ultrasonic clean the stripped bolt or pull the spring to remove all the carbon. 
• If you over lube your bolt it will just get dirty faster. Only run oil on the contact points where the bolt shows wear. 

By the looks of your pics the bolt is losing the empty case before the ejector has a chance to kick it out. I would start by replacing the extractor spring and detail cleaning the extractor. 

I'll double down on this.  Experienced FTE with what I characterize has my competition PCC.  Observed the following:

1. The bolt, itself, was gummy.  That can't be a good thing.  I do clean after every range session or two, but I'm clearly overlubing.  You don't need much lube for a 200 round range session.

2. Disassembled and cleaned the bolt:  lots of buildup

3. The ejector spring was about 10% shorter than a new MK10 spring.  I'm going to reassembly with the used spring; based on the general condition of the bolt it's just as likely that the FTE were caused by buildup rather than the spring, itself. 

Results: I ran 60 rounds through the SBR today as configured above (bolt thoroughly cleaned, lightly lubed, and the shortened ejector spring retained).  At this point I'm inclined to think the 10% ejector spring shortening is not to blame for the FTE.   I did find buildup under the extractor hook and removed same.

 

I am trying to use up a large amount of WSF in my reloads and experimenting with both 125gr and 147gr bullets.  WSF is not the cleanest and if the dirt buildup persists I'll return to SP, but I hate to see 12+ lbs. of WSF just sitting in the cabinet from 3 years ago.  The suppressed HD SBR will only see low round counts of factory ammo.

 

I have a US cleaner, but with pipe cleaners and such you can clean the bolt pretty thoroughly.  I'd suggest anyone with and CMMG invest in the simple tool for R&R of the ejector pin, but that's just me.

Edited January 18, 2021 by RickT

[JDIllon]

On 11/4/2020 at 2:22 PM, MoRivera said:

Same with me when it comes to the ejector and extractor pins, made my RDB completely reliable with two different barrel lengths and buffer weights.  But I've also only shot factory stuff and reloads that are a bit hot.  I really like this thing just wish the Endo-mag/Pmags had an extended version.

You can run Glock mags with a couple of mods to the Stern adapter. See my thread! Works great. JD 

[Pluder]

Awesome, thanks for all the advice guys! 

[MoRivera]

On 1/17/2021 at 8:26 AM, JDIllon said:

You can run Glock mags with a couple of mods to the Stern adapter. See my thread! Works great. JD 

I was looping at that before yeah.  I just want to keep the Stern the way it is as I still use it with my other blowback uppers.   Or at least I did when ammo wasn't so costly and I used to shoot regularly!