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.

So, If you are NOT doing that, most of this should NOT concern you.

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.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.545bolt-308ejectorHole2


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.