CMMG Guard tuning (9mm)

[NPDriver]

I finally got around to getting my FM Products mag release filed down to the right height. I must have had the mag release in & out a dozen times. It's cycling like butter at this point. It really didn't take a lot material removal, in fact I was worried I'd take too much off and have to build up with some weld & start over. Looking at the distance between my mag stop and the flat spot on the magazine I'm only looking at a spacer slightly thicker than 1/16". I was thinking maybe a piece of that rubberized refrigerator magnet. Easy to cut, holds its shape and already has adhesive on it. LRBHO still works on every mag I've finished. I was concerned I might have to modify it also after lowering the magazine height.

[MemphisMechanic]

17 minutes ago, NPDriver said:

I finally got around to getting my FM Products mag release filed down to the right height. I must have had the mag release in & out a dozen times. It's cycling like butter at this point.

 

You removed material from the top of the mag catch to let your Glock mags sit lower in the gun, correct?

 

Quote

Looking at the distance between my mag stop and the flat spot on the magazine I'm only looking at a spacer slightly thicker than 1/16". I was thinking..

 

You lost me here. I assume you’re talking about the underside of the ejector on most guns when referring to a “mag stop” ?

 

(You don’t mention which lower your’re using.)

 

Edited April 6, 2019 by MemphisMechanic

[NPDriver]

Foxtrot Mike's lower has a mag stop milled into the side of the magwell much like the Guard lower. That along with the LRBHO design where what sold me on their lower. Their excellent customer service was also a factor. I was asking some questions about the mag stop, (it was hard to see in photos) on their Facebook page and had the VP of the company send me a friend request so we could communicate more easily through messenger. 

 

Yes, the top of the mag release needed a bit of material shaved off to get the magazine to the correct height. I just filed it away a little at a time until I got it perfect. 

Edited April 6, 2019 by NPDriver
Photo

[Empireian]

I'm troubleshooting my build.

 

It consists of a cmmg glock mkgs lower and 8" barrel and bcg, aero xl upper, bcm a5 tube with brace, kynshit blitzkrieg 9mm buffer, tubb spring (tried 556 and 308) and bcm mcmr handguard and various parts from a cmmg lpk.

 

Most important a hiperfire edt heavy gunner installed with hiperfires pin set.

 

The secondary sear on that trigger was worn down within 300 rounds resulting in fire on reset if you ride the trigger forward but if I install the trigger in a different lower without a safety or grip it seems to work fine still. Hiperfire says they will send me a new hammer and disconnector but do you guys think it was a bad heat treat or is there something wrong with my build? I'm gonna borrow calipers to see if it complies with hiperfires pin specifications. 

 

I like the edt a lot and don't want to have to switch to a more expensive geissele ssa in this and future guns but I feel I may have to.

Edited June 9, 2020 by Empireian
Note

[Empireian]

Hiperfire sent me a new trigger, this one hasn't failed yet after 6 magdumps.

 

They mentioned they are updating their disconnector soon which may improve reliability in pccs.

 

I switched from ets to glock magazines and now I'm having overinsertion issues, going back and forth with cmmg support now lol. 

Edited July 1, 2020 by Empireian

[meeesterpaul]

my 2021 Resolute in 40SW started having ejection trouble. Spent rounds were getting tangled up with the incoming. Tried different loads but in the long run a major cleaning darn near took care of it all. I did have 1 tangled up round in a 3 Gun match the other day. Otherwise it was running potent loads of about 1225fps x 175gr at 200 yards. I had switched the buffer weights back to the stock weights which added up to 1.8 Oz after having replaced one of them with a piece of Delrin so I could run light loads. After a thorough cleaning and then relubing with Hornady One Shot it's running light loads with the original buffer weights. Those light loads are about 130 in my pistol and low 150PF in the CMMG. They feel embarassngly soft. Everything ejecting is nice. 

 

 

 

Edited December 1, 2021 by meeesterpaul

[MoRivera]

I recently had to replace the extractor spring in my RDB setup...this was a Sprinco extra power unit that had about 1500-1800 rounds on it.  When I took it out it was kinked and compressed.  Was getting a lot of FTE's with the shells either stove piping or getting jammed up into the charging handle channel.  Have replaced with a new Bravo Company enhanced unit and included the rubber insert just to hopefully help keep its shape.  Ejector spring seems fine, but it's strange how this system beats those springs down.

 

I find that mine runs best with an H2-equivalent Armaspec captured buffer system.

Edited December 1, 2021 by MoRivera

[RickT]

My 9mm is hard on ejector springs (CMMG says to expect 1000 rounds), but I get a bit of advanced notice as the ejection pattern starts to change.  If I press my luck I'll get FTE.  Running a hydraulic buffer in an A5 tube FWIW - great setup.

[troupe]

I shot a CMMG today at the range and it was in 10MM. I was shocked at how soft it shot. Did not expect the recoil impulse the pistol put out.

[MoRivera]

Okay so some strange recent issues.

 

In addition to my rifle upper, I have an 8" barreled upper that I use on an SBR for a short/suppressed PCC (still waiting on an Omega 9K).  I've had it for a while, actually got it with the BCG before getting an extra 16" barrel.  Recently been getting a ton of FTE's using the 8" upper, with the brass getting all caught up inside, stovepipes, what looks like double feeds.  Couldn't get through 2-3 shots before it would happen again.  It was happening before with this short configuration and I thought it was the ejector/extractor springs so I changed those out, been about 475 rounds since the change.  Was it the magazines (Endomag P-mag inserts)?  Are the springs going already?

 

Took the short upper to the range and was having all hangups...took the BCG out and put it in the longer-barreled upper, put that upper one the same lower and buffer...no problems!  So it's specifically that shorter-barreled upper.

 

But here's the weird thing...the problems with the short barrel have only been happening since I put it in an Aero XL upper receiver...the SAME KIND I have for my longer upper, and which runs like a champ with same BCG, same mags, and same everything else.  When I had the 8" barrel in another cheap billet upper receiver before (like a Midway Stoner house brand) while I was experimenting, no problems.  I put it in another Aero XL because it ran so well in my long version.....so WTF?!

 

Maybe something inside the second Aero XL upper was out of spec, or maybe some sort of headspace issue...the barrel extension seemed to seat just the same between the longer and shorter ones, and again the shorter was working just fine with an earlier cheaper biller receiver.  I took off the barrels and put it in an extra unused Andersen .458-port upper I had just laying around to see if it will run better.  Will give it a try, but at some point may just go back to that cheap house-brand billet reciter if it stays reliable.  Weird.

Edited February 20, 2022 by MoRivera

[MoRivera]

Crap....I'm wondering if the culprit is, of all things, this...

 

 

That's on the receiver for the 8".  The one on my longer barrel looks like this....

 

 

...and I remember now specifically that I folded it over that way.  Maybe that was just enough to impede the ejected round and keep it from exiting cleanly.  I'm going to fold it over on the new Andersen upper and see, but I should probably just remove these doors anyway, especially on the longer one since that's more of a gaming gun.

 

ETA: took the doors off both, so we'll see if that makes a difference.  Cleaned out the extractor and the spring still looked good.  From more reading up, it seems that it could be a headspace issue, especially on the 8".

 

Quote

What I don’t understand is why the system kills ejector springs when 5.56 guns don’t.

 

Best I can figure because the bolt is spring loaded in the "unlocked"  position and held closed by the recoil spring, the ejector is never fully compressed. When the round fires the spring gets a sudden acceleration to the rear for the distance of the difference between the width of the extractor groove and the case rim.....My thoughts being the case never fully seats in the bolt pocket but is pushed against the extractor by the ejector when the bolt is closed.   When fired the case comes back and hits the bolt face. Way more stress on the spring  that with a traditional locked breech system. as it basically cycles 4 times every time the weapon is fired.

 

Edited February 21, 2022 by MoRivera

[MoRivera]

Some more notes about the head spacing and the nature of the ejector, probably already posted elsewhere but anyway.....

 

 

https://c3junkie.com/?page_id=280

 

Quote

What I will say is that this (fixed ejector like on blowback AR9's) is how the CMMG RDB should have been brought to market.  All of my reliability issues have gone away moving to the fixed ejector.   On the ejector spring issues page I briefly mentioned about having a custom SS barrel made where I had all the headspacing slop removed.  I had DeadShot barrels do this work for me.  He was not familiar with the CMMG RDB design and so I shipped him a 9mm CMMG BBL / BCG as well as a SS blowback barrel and standard 556 extension to mate with the blowback barrel.   He agreed that headspacing was excessive on the CMMG barrel.  

 

And headspace:

 

 

I noticed that I have a bit more of this on my 8" barrel than on my 16" one.  So that may contribute to why I'm seeing a lot more malfunctions with the 8".

Edited February 21, 2022 by MoRivera

[MoRivera]

Okay so...going off the rearward movement/headspacing as demonstrated above in video, I measured it comparing my 16" barrel and my 8" barrel.

 

With the 16" I got about 2mm of total rearward movement.

 

Withe the 8" barrel I got 1.5mm of total rearward movement.

 

I was expecting the opposite!  Since I'm experiencing all the FTE's and stovepipes with the shorter barrel, I figured it would have more excessive headspace and not have as much leverage on the expended shell to flick it out.  Does the 8" not have enough headspace?

 

Just ejecting a dummy round/snap cap by hand, the longer barreled upper at least drops it out the ejection port.  The shorter one almost never does, just lets it trundle out the bottom/magwell, no matter how fast I try to rack back. 

 

So I'm still stumped as to how the shorter barrel upper is so unreliable, whereas come to think of it I've never had a hangup with my longer-barreled upper, through a bunch of matches and a lot of rounds - except maybe one issue at the end of the magazine, which I attribute more to the magazine (P-Mag Endomag insert).  That's kind of fortunate for me as my longer-barreled one is for matches, and the shorty is more of a fun gun or maybe for a suppressor, so the longer gets more use.  But still, the difference is worrisome.

 

Question...is a lighter buffer proffered for the short barrel because of less rear pressure?  I was using an H2 equivalent buffer, then switched to an H1 for the 8" but still having these problems.  Was wondering if I should go down to a Carbine-weight to ensure better rearward velocity/ejection with a shorter barrel?

Edited February 25, 2022 by MoRivera

[MoRivera]

Inspected the bolt closely and saw this.....

 

 

 

Chipping on three lugs of the bolt right at the angled cuts where it engages the barrel extension and rotates.  Wondering if that is having anything to do, but as mentioned no problems noticed with the 16" barrel.

 

Others have experienced, documented here.....

 

https://www.ar15.com/forums/Armory/CMMG-RDB-9mm-Chipped-Bolt-Lug/48-511703/

 

I've written to CMMG to inquire and hopefully replaced.  I the meantime I have a 'repair kit' which comes with an extra bolt and extractor, etc.  Will replace but wondering if it will happen again.

 

Also wondering if I should go with some grease on the lugs, but concerned that may gum up with fouling even more.

 

 

Edited February 25, 2022 by MoRivera

[MoRivera]

Update: went to the range with just the 8" upper, in the Anderson receiver, no port cover, new replacement bolt.

 

Ran like a top, 200 rounds and change, not a single hang-up or issue, bolt locked open.  115-gr factory Blazer Brass, and 147-gr subsonic through an Omega 9K suppressor.  H1 buffer.  Fast strings of doubles and triples, even some tac reloads which would be the death knell before.  Ejection was a consistent and healthy 4:00-4:30.

 

So it seems so far (again only 200-250 rds) that some combination of above has done the trick, but I can't help but wonder if like before it's fine in the beginning but will soon just all go to hell again.  I'm still pretty confident in my longer upper getting through a match as it has each time, but the shorter one for 'fun' matches and maybe even some home defense with suppressor...still wary.  Can only tell if it's really fixed if it holds together up to 1000 rounds or so.  Last round count where it went bad after refreshing the extractor and ejector springs was only around 450.

 

Will see if CMMG will still offer some sort of replacement for the damaged bolt.

 

At least from the looks of out (again only first 200+ rounds), the lugs/edges seems to be fine but you can see wear the brunt of the contact wear is.  Except the second and third lug after the extractor seem to have their leading edges at an angle compared to the rest.  Don't know if that's from initial wear or by design.

 

 

Edited February 26, 2022 by MoRivera

[MoRivera]

Good news!

CMMG got back to me and will be sending me another bolt, so at least I'll have an extra backup.  Much appreciated to them for responding.

[MoRivera]

Also want to note that for the extractor...they sent me a new one when I requested a while back, and also in the bolt that's part of the repair kit....it has the o-ring whereas before I think it was only the spring and center post.  I guess this holds up better against compression and kinking so it's what they recommend from now on.

 

 

Edited March 13, 2022 by MoRivera

[MoRivera]

Interesting new video....

 

 

 

 

RDB on bottom: 

Edited March 29, 2022 by MoRivera

[Texaspaul]

I agree the RDB is softer/spread out recoil, but the reliability of the ejector and the lug issue are issues that CMMG does not seem to acknowledge.  I would love to run my CMMG at Steel Challange but due to dependability issue it sits in the safe. I contacted them once and asked if I could just buy the ejector spring $5.00 part and they told me I need to buy the rebuild kit $35 to get the spring.  I would replace the spring every 1,000 to 1,500 rounds to get the soft recoil and reliability, but not for $35!

[MoRivera]

17 hours ago, Texaspaul said:

I agree the RDB is softer/spread out recoil, but the reliability of the ejector and the lug issue are issues that CMMG does not seem to acknowledge.  I would love to run my CMMG at Steel Challange but due to dependability issue it sits in the safe. I contacted them once and asked if I could just buy the ejector spring $5.00 part and they told me I need to buy the rebuild kit $35 to get the spring.  I would replace the spring every 1,000 to 1,500 rounds to get the soft recoil and reliability, but not for $35!

Yeah, to be frank I'm kind of on my last straw with this system when it comes to reliability.  As it stands right now I have a new bolt in place (and them sending me a replacement), and I'm hoping that both the newer ejector and extractor spring/o-ring washer will prove to be the trick.  But if it starts acting up again under 1000-1200 rounds, then I've enough and I'll go back to my regular blowback setup which I had gotten quite pleasant with a combination of hydraulic buffer and spring setup, and was dead-nuts reliable.

 

I think it's a very clever system and certainly has its benefits on shooting and traceability, but I get the feeling that it's putting opposite forces on the traditional AR-bolt setup that it wasn't really designed for.  And then instead of a longer case to 'swing' out with more torque, you have a squat little case which just stays around until it sort of trundles its way out at the rear.  But I really hope now it stays reliable.  I can handle a spring replacement here and there every 1000 rounds, but not the mystery of if it'll make it through a match along the way.

[Dan Sierpina]

18 hours ago, Texaspaul said:

I agree the RDB is softer/spread out recoil, but the reliability of the ejector and the lug issue are issues that CMMG does not seem to acknowledge.  I would love to run my CMMG at Steel Challange but due to dependability issue it sits in the safe. I contacted them once and asked if I could just buy the ejector spring $5.00 part and they told me I need to buy the rebuild kit $35 to get the spring.  I would replace the spring every 1,000 to 1,500 rounds to get the soft recoil and reliability, but not for $35!

The ejector springs are available. I bought 10 of them.

Spring, Ejector, Mk10 | CMMG - AR 15 and AR 10 Builds and Parts

[Texaspaul]

Just ordered 5 - got a laugh out of the "build to order" delivery and they will bill card now no matter when they ship???

 

This will be fun to follow 

[Texaspaul]

Update - order shipped - yahoo!

[Texaspaul]

Shot the CMMG in a SC match today on three stages while running my NFA Scorpion mag direct blowback on three stages. CMMG defiantly softer but also slower bolt action. I did not outrun the bolt but just felt slower.  Did not have one issue with CMMG but had two FTF on the NFA.  Tried two different bullets on the CMMG 115 Berry's 3.2gr Clean shot and SRP and 147 Berry's 3.2gr Clean Shot and SRP. The 147 moved the bolt better and the 115 seemed sluggish.  Both guns are running 4.5 barrels with shrouds to 16 inches.  CMMG seems to work better with higher power factor ammo. more to follow!     

[MoRivera]

On 4/1/2022 at 5:31 PM, Texaspaul said:

Update - order shipped - yahoo!

Mine too.  Good to at least have some more on hand....just hope I don't have to replace every other match!

 

They had actually sent me two before when I mentioned some of the failures I was getting, but still good to have more.

Edited April 3, 2022 by MoRivera