CMMG Mk10 10mm forceful ejection

[Lakotah]

I recently purchased a 10mm in Coyote. After 15 rounds this is the results:



I called CMMG and was told the typical this is normal wear. I have seen alot of posts all over the internet about this wear not being normal. I was told to get the Mk10 heavy buffer. Been over 4 weeks now and it's still back ordered. Just not sure if just going to a heavier buffer is going to fix this problem.
Does anybody have any ideas how to fix it? I really liked shooting it, but with only 15 rounds through it and already this much wear, I really don't want to shoot it anymore before I can attempt to fix it.
I currently have the H3 buffer installed. I was shooting Sig Elite 180gr. Full Metal Jacket. By the Power Factor chart I was within the limits. So I have no idea why this is happening.
If anybody can give me some guidance, part numbers, ideas, or if they have something that they upgraded to that is working well, please let me know. Thanks in advance

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[TTSX]

Heavier buffer spring. I have a home brew .45acp with cmmg RDB parts and I use a sprinco blue and standard carbine buffer. Works like a charm. Springs don't add reciprocating mass, are cheaper and will slow your bolt down just the same.

 

[Alexx1401]

Ditto another spring. They are cheap. CMMG has a great rep for standing behind what they make. If this becomes a real problem suspect they will repair or replace but another spring would be a quick cheap thing to try.

 

[Member 34319]

This is a known issue with the RDB systems CMMG uses. There is a good thread on an AR based forum talking in great detail about this. I want to say someone from CMMG was on that thread too.

The CMMG radial delay set up uses a similar ejector to an AR15, which is a plunger on the face of the bolt. It however does not have a fixed ejector like most straight blow backs do. By design it can't really, as the AR upper receiver wasn't really made to have this. Cudos to the AK for knowing that it is important to ensure cases get flung far away from moving parts.

Anyways. Because there isn't anything to hit the case, it rides in the bolt until it hits this part of the upper. The case literally rides at an angle against the inner right side of the receiver, till the port, then if it the extractor is still gripping the case, which it usually does with lightweight pistol cases, it rides in the bolt reward at an angle, as part of the case is now being pushed forward by the ejector plunger. Sort of tried to show this in the attached picture. This part of the ejection port is technically the ejector, even for ARs in rifle calibers.

Some have messed around with the length of travel the bolt can move in the buffer tube. Forcing the bolt to stop before the case hits this spot. This basically forces the cases reward momentum to act as the ejector. Bolt comes to a complete stop, brass and extractor give letting the brass go finally. However pistol cases don't have the most mass sometimes the extractor just won't let go, so without that wall of the port door hitting the case, the case just stays put on the bolt and goes back forward causing malfunctions. Others have modified the plunger to try to help. Some if I recall even messed around with different extractors and springs and such.

Most just come to terms that this is how the RDB works and call it a day. Mine has had tens of thousands through it. It's basically shot peened in that corner now. Yes it's cosmetically ugly. I don't really care about that. I just like that it's reliable. Up until it wasn't because of their use of the plunger ejector vs fixed. 9mm cases don't weigh a bunch. Trying to move a small amount of mass in another direction than a big chunk of metal (the bolt carrier) with out a sharp change in direction via some form of fixed ejector is the CMMG RDBs Achilles heel.

Reno

 

[Member 34319]

Pictures.

 

[Caveman Jim]

What marks or damage does your brass have? If dings are prevalent then super glue a small piece of fuzzy Velcro where the brass contacts the upper housing, like so.
Good luck!

 

[Lakotah]

Thanks for the info, it would be easier to accept if it wasn't just 15 rounds. The back of the port had gouges in it, it isn't just scraping or removing coating. I can only imagine with it's gonna look like after a couple hundred or 1k rounds. That's why I wanna try n fix it now before it gets to a warranty issue. Had to wait 6 months to get it, don't want to have to wait another 6 to get it fixed

 

[Lakotah]

I have Velcro on my AR so I know that would help but there isn't any marks on the deflector.
It's like it's contacting the back of the ejection port and then ejecting.

As for rounds. I honestly didn't know there was an issue till I got home and started cleaning it so I never looked at the ammo. I don't reload so I didn't bother keeping it. I just threw them in the bucket.
Next session out after I get something to try I will definitely collect the rounds for inspection.

 

[Member 34319]

Thanks for the info, it would be easier to accept if it wasn't just 15 rounds. The back of the port had gouges in it, it isn't just scraping or removing coating. I can only imagine with it's gonna look like after a couple hundred or 1k rounds. That's why I wanna try n fix it now before it gets to a warranty issue. Had to wait 6 months to get it, don't want to have to wait another 6 to get it fixed

I'll say it now, if you send it in, they will likely give you a new upper. However, it will likely still do this. It's just a design flaw of trying to do something different.

 

[Lakotah]

From what I'm read, you are correct so I'm trying to come up with a fix myself and with the help of the great people on this board.

 

[Member 34319]

From what I'm read, you are correct so I'm trying to come up with a fix myself and with the help of the great people on this board.

I'll follow the thread. If you do I'd be interested.

Thus far I've really enjoyed mine. However, I've learned the slight difference in recoil and sound suppression isn't worth the loss of a fixed ejector.

 

[TTSX]

From my .45 homebrew using an aero precision XL upper. More than 100 rounds but less than 200. No significant impact wear and the bolt does not travel far enough back to use the ejection port as a fixed ejector possibly due to the enlarged port the XL model uses

 

[Lakotah]

Ok went back to the range and looked in the bucket. There were only a few additional rounds of 223 in the bucket. Just had a little rain in it. Anyway, I don't see any swelling or bulging in the rounds. Did see a little notch in the top of the casing though.

 

[Member 34319]

Look along the base wall on the exterior and see if there is a dent of any sort where the case might be hitting the ejection door wall.

If you have a newer smart phone with slow motion feature, perhaps have someone film the port while shooting it and get a better idea of what's going on with yours. It might help find pin point a starting place you can tweak to assist with the ejection patterns.

 

[Lakotah]

Ok, came home for lunch and cleaned the brass. Here's the bottom of the cases.

 

[Lakotah]

3 of the 4 rounds had this mark on the side. It's magnified quite a bit. I really couldn't see it with the naked eye except with a bright light and rotating it at an angle