CMMG FTE

[Texaspaul]

Mine now sports a 5" barrel with a Taccom shroud pined - good shooter - one issue I have is bolt sometime gets stuck and have to bounce the stock to release the bolt?  Any thoughts?

[MoRivera]

14 hours ago, Texaspaul said:

Mine now sports a 5" barrel with a Taccom shroud pined - good shooter - one issue I have is bolt sometime gets stuck and have to bounce the stock to release the bolt?  Any thoughts?

Ouch, stuck forward?  It could be case expanding and sticking.  Maybe the bolt lugs hanging up on a bur?

[Texaspaul]

Actually sticks with no bullet in chamber - when i pull back charge handle to lock bolt - it will stick. if i give the butt a smack from behind, it free up. it happens ever so often not always.  I really want to like this gun but there seems to be small issues like it was not fully tested before being offered to public. 

[MoRivera]

2 hours ago, Texaspaul said:

Actually sticks with no bullet in chamber - when i pull back charge handle to lock bolt - it will stick. if i give the butt a smack from behind, it free up. it happens ever so often not always.  I really want to like this gun but there seems to be small issues like it was not fully tested before being offered to public. 

So it stick's back as opposed to when it's forward?  Then something's binding, maybe in the buffer tube, or maybe the buffer retaining pin?  Or the spring and buffer are letting it ride back too far and get hung on the hammer...or the spring is binding up.  Maybe try with a different buffer/spring first.  Did you build the upper?  If yes, sometimes the buffer tube isn't screwed in far enough and lets the retainer pin stick up too much and bind.

Edited April 8, 2020 by MoRivera

[Texaspaul]

no sorry it sticks in the forward positron when I try to pull the handle back the bolt locks in the breech

[MoRivera]

4 hours ago, Texaspaul said:

no sorry it sticks in the forward positron when I try to pull the handle back the bolt locks in the breech

Ah I see.  So maybe it could be something in the lugs between bolt and barrel, or maybe something binding in the rear, or even between the bolt and carrier.  Still may want to check the buffer retaining pin, but also have you given it a thorough cleaning?

[RickT]

I've done a lot of testing since my original post.  I have both the original bolt with a new extractor (from CMMG) and a complete bolt assembly from CMMG.  The original bolt works better than the complete bolt assembly.  Most recent test with the original bolt: FTE on the second round only which I suspect is related to the stiff magazine springs since the remaining 28 rounds cycled normally after clearing the stoppage.  The same attempt with the complete new bolt: FTE on rounds 2, 4, 6, 10 of a 30 round mag.  I stopped at 10 rounds.  I'm going to load up four mags, my last waste of 120 rounds of factory ammo, and see how many failures I get with the original bolt.  Frankly, I'm about to give up on this thing.

[MoRivera]

Crap.  Would there be any way to take detailed photos of each and post?

[RickT]

I'll try to take some pics tomorrow.  Meanwhile, I decided to measure ejector spring length.  One of the springs was really stubborn probably due to blowback, but a surprise after so few rounds.

  Spring                                                        Length

1. Unused spare from CMMG                  0.947"

2. Original spring in original bolt             0.887"           This bolt produced numerous ejection errors

3. Replacement spring in original bolt   0.808"            This spring installed together with extractor assembly.  Obviously a lot shorter than the unused spring

4. Original spring in a new bolt assy.      0.895"            

#3 has probably seen the heaviest use and I don't know if that degree of compression is to be expected, but #3 also performs better than the brand new bolt assy.  Best of all is the bolt out of my original 16" guard which rifle BTW has never experienced an FTE - go figure.  

 

The 120 round test should be telling.  I did replace the spring in the original bolt; not only was it considerably shorter than a unused spring, but it had a slight bend on one end which probably explained its reluctance to drop free of the ejector bore.  In addition to the factory ammo I've also reloaded a few rounds with more powder to see if long term (if there is a long term) I can get the gun to run a bit cleaner.  No problem cleaning readily accessible surfaces and the locking lugs, but I don't want to be taking the bolt apart every 300 rounds.  FWIW anyone with a PCC that uses an AR-style bolt is crazy not to have a tool to remove and replace that pin.

       

Edited May 10, 2020 by RickT
Additional info

[MoRivera]

Very strange that a newer version is less reliable.  Could it be a compatability issue with the barrel lugs?

Edited May 10, 2020 by MoRivera

[RickT]

I've run the new 8" upper on my original glock lower as well - still see the same problems.  At least this has convinced me to buy a 300 blk upper for home defense; for the time being I'll use the same SBR lower for competition and HD>

 

And the final test with a new ejector spring was a big fail with factory ammo - FTE every 2-3 rounds until I gave up.

Edited May 10, 2020 by RickT

[chipsher]

I've given up on the RDB for now. Love the feel of the action but has never been reliable for me. Best I could get was one or two FTEs in a match of 125 to 150 rds. I've tried different recoil springs. Tried cutting a coil at a time to see if I could get a change. Different buffer weights, hydraulic buffers of different weight. Took sharp edges of extractor. Ejection port to barn door size. Different bullet weights, different charge weights. Replaced ejector and extractor spring with extra power chrome silicon (often). Mine worked best with a 115gr bullet, fairly hot load.

 

I work as a Tool and Die Engineer and I work with part ejection all the time and this one beat me. I think the only real solution is a fixed ejector, but for a compitition gun I don't want any non-standard parts. I can't believe CMMG missed this. Or I just shoot too much. I burned over 8,000 rds. trying to get this to work.

 

 

Back to the JP and Scorpion 100% reliable.

Edited May 11, 2020 by chipsher

[MemphisMechanic]

@chipsher you tried an extra power BCM-style extractor spring in the gun, I’m sure?

[MoRivera]

6 hours ago, chipsher said:

I've given up on the RDB for now. Love the feel of the action but has never been reliable for me. Best I could get was one or two FTEs in a match of 125 to 150 rds. I've tried different recoil springs. Tried cutting a coil at a time to see if I could get a change. Different buffer weights, hydraulic buffers of different weight. Took sharp edges of extractor. Ejection port to barn door size. Different bullet weights, different charge weights. Replaced ejector and extractor spring with extra power chrome silicon (often). Mine worked best with a 115gr bullet, fairly hot load.

 

I work as a Tool and Die Engineer and I work with part ejection all the time and this one beat me. I think the only real solution is a fixed ejector, but for a compitition gun I don't want any non-standard parts. I can't believe CMMG missed this. Or I just shoot too much. I burned over 8,000 rds. trying to get this to work.

 

 

Back to the JP and Scorpion 100% reliable.

The more I read it just seems that the action of the bolt and carrier being pushed back by the round is beating the ejector and spring up, as compared to the carrier being pushed away by an AR's gas and pulling the bolt back.  Might be a fundamental flaw in the design approach that's only really become evident through lots of firing.  I hope mine holds up because I really like the way it shoots.

Edited May 12, 2020 by MoRivera

[chipsher]

No Sir, didn't try the BCM Spring.  I tried Wolf and Wilson extra power springs. I wanted everything to be as standard a component as possible. And I have a stock of these from shooting 3gun.

Edited May 12, 2020 by chipsher

[RickT]

5 minutes ago, MoRivera said:

The more I read it just seems that the action of the bolt and carrier being pushed back by the round is beating the ejector and spring up, as compared to the carrier being pushed away by an AR's gas and pulling the bolt back.  Might be a fundamental flaw in the design approach that's only really become evident through lots of firing.  I hope mine holds up because I really like the way it shoots.

That may be the situation, but the failures I'm seeing out of the box are likely a manufacturing issue.  FWIW I've never had an FTE with my early S/N 16" 9mm CMMG RDB.  Heading the range tomorrow (again).

[chipsher]

My guess is the bolts initial reward thrust is at very high speed and spring coil binds by it own inertia. Or maybe when the bolt first get slowed by the carriers weight. Probably both like a one two punch. One other odd thing I noticed watching the action operate. During a jam it looks like the empty never leaves the bolt face, even though there was still a pretty good ejector spring. Never saw it bounce around, always the same type of jam. A high speed camera would be nice.

[RickT]

8 minutes ago, chipsher said:

My guess is the bolts initial reward thrust is at very high speed and spring coil binds by it own inertia. Or maybe when the bolt first get slowed by the carriers weight. Probably both like a one two punch. One other odd thing I noticed watching the action operate. During a jam it looks like the empty never leaves the bolt face, even though there was still a pretty good ejector spring. Never saw it bounce around, always the same type of jam. A high speed camera would be nice.

Agree on the bolt face.  When the ejector does its job it slings the cases several feet, but these are hard jams with the case pivoting around the extractor and the next round wedged in there. 

[MoRivera]

9 minutes ago, RickT said:

Agree on the bolt face.  When the ejector does its job it slings the cases several feet, but these are hard jams with the case pivoting around the extractor and the next round wedged in there. 

Also, when cycling an unfired round out manually it doesn't seem to really fling it out, the round often just trundles down out the bottom or even stays in there.  Ejects fine while firing, at least for me so far.

 

What gets me about this is have we heard of people's Banshee's breaking down like this because technically it's the same system and key parts, right?

Edited May 12, 2020 by MoRivera

[ziebart]

I think that has to do with the shorter barrel being better suited to normal ammo. My 16" hates hot ammo, but seems to work okay now on downloaded stuff

[MoRivera]

I've got the 8" going on both uppers.

Edited May 12, 2020 by MoRivera

[ziebart]

There goes my idea

[RickT]

9 hours ago, MoRivera said:

Also, when cycling an unfired round out manually it doesn't seem to really fling it out, the round often just trundles down out the bottom or even stays in there.  Ejects fine while firing, at least for me so far.

 

What gets me about this is have we heard of people's Banshee's breaking down like this because technically it's the same system and key parts, right?

My upper is a complete CMMG unit so there should be no question of headspace (which frequently gets raised in the context of ejection issues).  FWIW I've had some spectacular fails with two Gen2 CMMG-branded mags.  Haven't seen this with the Endomags using Gen 3. (That picture wasn't staged!  That's what they looked liked after the stoppage.  Can't imagine how that happened)

 

I always insert a full mag with an open bolt and the next round definitely gets beat up by the bolt passing over it including scratching through the coating on that round.  Some folks have smoothed out the lower lugs a bit, but I don't think this behavior is necessarily relevant to the ejection issues.

Edited May 12, 2020 by RickT

[mrd]

12 hours ago, ziebart said:

I think that has to do with the shorter barrel being better suited to normal ammo. My 16" hates hot ammo, but seems to work okay now on downloaded stuff

 

I have a 16" and it seems to like ammo in the 145-160 PF range., at least in regard to accuracy. I have no feeding issues with any loads, but the rifle have only run about 1500 rounds. 

[RickT]

I did a series of mini tests involving all three of my lowers and both 16" and 8" CMMG uppers.  One of the two new configuration bolts worked well with two varieties of factory ammo and one of my reloads.  The only possible explanation is that when I changed out the ejector spring in one of these bolts I swapped the labelled plastic bags I keep the bolts in and never tested the bolt with the new spring.  The other bolt was producing light strikes with both ADM and BCM lowers.  Match today so I'll find out if I can get through 100 rounds with this 8" upper.

 

I am now officially out of factory ammo;  good thing I reload, but I really need some factory for testing purposes.