CMMG Guard Ejection Issues

I have noticed that if you try to slow down the cyclic rate on the 9mm CMMG Guard, the faster it kills ejector spring.***

*** UPDATE ***
I had initially thought it was just people like me trying to slow the cyclic rate down running into these issues but I’m hearing more reports of others having ejector spring failures not running my preferred buffer/tube/spring arrangement.  Most people seem to be having failures around 1K rounds as well.

First of all, I have 2 CMMG Guard 8″ Barrel and BCG combos.

The first set I got came with a dual feed M4 barrel extension and 1/2-36″ threads.

The second set I got came with a single feed ramp barrel extension and 1/2-28″ threads.

I had done all my tinkering trying to slow the full auto cyclic rate down and started running into failures to eject…about to give up, I tried the BCG from my second order and it ran everything fine.  I later determined that the ejector spring was different as well as exctractor design was also different on the second BCG.

CMMG told me that the .45 Guard uses a proprietary ejector spring that shorter but stronger than a standard 556 ejector spring which the 9mm Guards initially shipped with and they later changed the 9mm Guards to use the same spring as the 45 Guard.

Pictured below is a Sprinco / 556 Ejector spring which is of typical length as others in the industry.EjectorSpring-Sprinco

Pictured below is the typical wire thickness of a 556 ejector spring.WC-thickness

Pictured below is the Guard 45, and LATER 9mm ejector spring. (I also believe the .40SW is using the same spring as well).GuardEjectorSpring-New9mm-45_10-2018

Below is the thickness of the Guard ejector spring.Guard-thickness

Comparison of some ejector springs.comparison

Putting the bolt in a vise and using a punch to compress it, I then used a trigger pull gauge to figure out approximately how much force was required for each ejector spring.
Below is a 556 Wilson Combat ejector spring.NewWC

Below is a new Sharps 556 BCGSharps556

New Guard Ejector springNewGuard

This is a worn out 556 ejector spring where I started to get malfunctions.Worn556

Worn out Guard ejector spring where I started having malfunctions as well

Note that in both cases where I started having malfunctions was at a slow cyclic rate.  These springs would still work if it was cycling faster.

Pictured below is a 5.45×39 bolt modified for the Guard with my own angles cut on the bolt to further delay the recoil which killed the ejector springs in less than 100 rounds.
Bolt was modified further to accept a 308 ejector spring.

308 Ejector on the left, 556 in the middle and makeshift ejector for the Guard on the right.

View of hogged out ejector hole

View of makeshift ejector installed in bolt

This worked but only for like 30 rounds before the 308 ejector spring was completely dead.
I gave up on the ejector in the bolt at this point and went to a fixed ejector done by another C2 manufacturer that has a prototype 9mm piston upper that uses a fixed ejector.
This has turned out wonderfully.

Note that I now see that CMMG is releasing a .40SW Guard and they have updated the ejector to be bit different from what we normally see:

What may also be different is that lugs may have a less aggressive angle as the 9mm Guard like what I was doing on my 5.45 bolts to slow the unlocking.

I have UZI bolts that have the bolt face opened up to work with .40SW and 9mm.  I’m hoping this new .40SW bolt may actually work for 9mm as well for those that want to slow the RoF and not have ejection issues doing so.

I just got my .40SW BCG and Barrel and the lugs clearlyhave less of an angle on the .40SW vs 9mm. 40SW-vs-9mm

Other than now having a black finish, the ejector is now rounded like seen in the pictures from the pre-release version above.
As a side note, I used to have a .45 ACP gas operated upper by RMW.  He cut the face of the ejector to have an angle to help kick the brass out at the correct angle.  I think CMMG is basically doing the same thing here but by making it rounded, they don’t have to deal with ‘indexing’ of the pin.

The length appears to be the same as a 556 ejector.Ejector

The spring appears to be same as what is used in the 9mm and 45 ACP Guard…shorter but stiffer than a standard 556 ejector spring.40SW-ejector-spring


Update 6/22/19

Put some more rounds through the .40SW bolt and measured the current strength of the ejector spring.  Almost 6 lbs.  In my experience, once you get under 5 lbs is where you will start having issues with the 9mm when using my preferred buffer / tube / spring configuration that runs in the 600′s.

However, I don’t know what effect the rounded ejector will have….the spring is already holding up better than anything else I have tested.  My modified 5.45 bolt with angles similar to the .40 SW killed the ejector spring in less than 100 rounds.  While it was a standard 5.56 ejector spring, I would still anticipate a factory Guard spring to die relatively quickly.

The rounded ejector may also continue to work properly once the spring gets under 5 lbs as well.  I won’t know till it has gotten to that point. Ejector-6-22-19

While not on the CMMG website (at the time of this writing), Optics Planet is selling a spare parts kit:

They list two kits at that link.  One specifically states it is for .45ACP while caliber is not specified on the other one but by looking at the part number it appears to be for 9mm.

Looking closely at the picture, you can see it includes the new rounded ejector.
mkg45-repair-kitI contacted CMMG about this and they confirmed that the rounded ejector is only being included for .40SW and .45ACP (which were previously standard 556 ejectors).  I also inquired if the rounded ejectors have shown to increase the ejector spring life and was just told that it ‘worked better’ for those two calibers.  Kinda vague and decided not to keep asking the same question over email.


Update 8/3/19 
40SW ejector spring is starting to fail now. Note that previously didn’t have any FTE’s in 40SW and now it is occurring in this caliber as well.  Indicating ejector spring is failing now.Ejector-8-3-19_2


Update 8/24/19 

In the ‘Guard’ thread someone mentioned trying an HK 416 ejector spring.  Never thought of that.


A lot longer than a CMMG or standard 556 ejector spring


I did do some searching and found this thread:

In that thread, someone stated the HK416 ejector spring is weaker than a standard AR ejector spring and he was right.HK416-EjectorSpringStrength40SWBolt


Although it is weaker, at 6lbs it should still function fine for .40SW.  I am interested in longevity.  If it stays at 6lbs for at least 2K rounds then it is an improvement.

Tested it today and ran (4) 30 round mags in full auto suppressed in 40SW and had zero malfunctions.

However, when I tried it putting the BCG in my 9mm upper I couldn’t get past one round without a failure to eject.

A couple things still to try.
1.  Maybe use a standard flat faced 556 ejector in the .40SW bolt since the rounded ejector is really for the larger cases.
2.  Some others have mentioned to put a rod inside the ejector spring to reduce the spring from binding.   I have some drill rod laying around and plan on trying that.

However it was a major pain to install that HK spring and ejector in the bolt and don’t want to mess with it till I get a bolt ejector tool to make it easier to swap the ejector and spring.