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Posted: 6/13/2023 7:30:42 PM EST
I'm trying to decide whether or not to invest further in the CMMG RDB system (not Dissent) and need to know the latest on the ejector spring issue - Is the Mk10 spring the answer? Or? Any other issues to know about?
VP
Link Posted: 6/14/2023 5:38:39 AM EST
[#1]
Not in my experience. The same thing happened to the Mk10 spring and now they replaced the bolt saying they’re subtle differences. The must be real subtle as I can’t find any.
Link Posted: 6/14/2023 8:02:07 AM EST
[#2]
I had a different problem with my early RDB system (not a complete CMMG gun, just barrel and bolt ).... the polymer helper for the extractor hardened and caused a problem... a reader on the Brian Enos site suggested an upgrade to a Bravo Company HD extractor spring that I made..I used it without a polymer plug, but do use an O ring....seemed to have cured the problem, but I really haven't shot it more than 500 or so before COVID closed down my PCC shooting, that I didn't re start...I switched to rimfire Steel Challenge... I may resurrect the PCC for Steel Challenge for grins
Link Posted: 6/14/2023 5:18:09 PM EST
[#3]
I bought a complete Banshee 5" pistol that I SBR'd. Took it out the other day and as usual it shot flawlessly. I generally run 147 subs in it suppressed.

I've heard of a few people having issues with extractors and/or springs but mine has been perfect for at least a few thousand rounds. If it makes any difference my version uses a standard AR lower and I run CMMG branded "Endomags". This has been an awesome setup for me as I can slap multiple uppers on this one, SBR'd lower. One of my favorites is an 8", dedicated .22LR upper with CMMG bolt.
Link Posted: 6/14/2023 6:57:04 PM EST
[#4]
I contacted CMMG customer service last night about poor ejection that recently started with my 9mm rdb and I received a series of emails this morning about it. I told him I purchased my kit in early 2020 and he said they have a updated bolt assembly and would send one to me. So we will see if this fixes it, what spring is in the new bolt, and if there is any change to the bolt itself.
Link Posted: 6/14/2023 7:18:32 PM EST
[#5]
So 2 updated bolt replacements rather than offering an Mk10 spring! I wonder when these became available and if they are being used in current production. CJofFL when was your bolt replaced?
Link Posted: 6/15/2023 5:33:15 AM EST
[Last Edit: CJofFL] [#6]
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By 9nzz:
CJofFL when was your bolt replaced?
View Quote


September July 2022 and I didn’t remember asking about the differences, but just realized their email said, “it may not be visually noticeable because we’re talking 10ks of an inch difference in length and a slight change of the angle on the lugs.”
Link Posted: 6/15/2023 8:39:47 AM EST
[Last Edit: SSTPAC] [#7]
When ever I get my bolt in I will mic them and see what the differences are.

I know on amphibian's web page he spoke about the bolt angles being different among the different calibers.

Also cmmg lists a short barrel and a long barrel bolt for 10mm so it seems they are fine tuning the lug angles for different applications.
Link Posted: 6/15/2023 9:09:18 AM EST
[#8]
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By SSTPAC:
When ever I get my bolt in I will mic them and see what the differences are.

I know on amphibian's web page he spoke about the bolt angles being different among the different calibers.

Also cmmg lists a short barrel and a long barrel bolt for 10mm so it seems they are fine tuning the lug angles for different applications.
View Quote

IIRC, for the H & K roller on ramp delay system, blocks with different angles are required to be installed for different applications/configurations.  And if the wrong block is used or the rollers are not kept properly lubed, damage to the receiver can result.

Similarly, KynSHOT sells a veritable plethora of specifically designed/tuned hydraulic buffers that are tailored to particular applications/configurations.  If the wrong hydraulic buffer is used, the buffer might go penguin.

MHO, YMMV, etc.  Be well.
Link Posted: 6/15/2023 10:25:25 AM EST
[#9]
@amphibian
Link Posted: 6/15/2023 10:46:52 AM EST
[Last Edit: Gotterdammerung] [#10]
CMMG recommends Winchester White box ammo (which is on the hot side) so I used WWB and similar ammo and had tons of failure to extract/eject issues after the 1st 1500 rounds or so. I replaced the extractor with the BCM HD extractor and that worked for awhile, then the MK10 ejector spring would wear out every 1000 rounds or so. I also noticed tiny brass filings in the lower and broke 2 cam pins. Finally, I switched to lighter ammo and all's been fine since. Mine likes the Magtech 115. I also use a kynshot buffer and Tubbs spring.
Link Posted: 6/15/2023 8:05:58 PM EST
[#11]
I have seen alot of reference to CMMG recommending buffer weights or ammo types but I havent seen anything directly from CMMG. On their website all I found was the RDB owners manual which referenced buffers in regard to 10mm loads, and a blurb about the action tuning kit for fine tuning to your loads or suppressed use. Can anyone link to where CMMG makes these statements?
Link Posted: 6/16/2023 7:53:58 AM EST
[#12]
CJofFL what all did you receive for the replacement bolt? Was it the complete bcg repair kit or just a replacement bolt only?
Link Posted: 6/16/2023 11:19:43 AM EST
[#13]
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By SSTPAC:
I have seen alot of reference to CMMG recommending buffer weights or ammo types but I havent seen anything directly from CMMG. On their website all I found was the RDB owners manual which referenced buffers in regard to 10mm loads, and a blurb about the action tuning kit for fine tuning to your loads or suppressed use. Can anyone link to where CMMG makes these statements?
View Quote
I think it may have been discussed in this old thread when the CMMG 'Guard' was announced back in 2017: https://www.ar15.com/forums/AR-15/-ARCHIVED-THREAD-Thoughts-on-the-CMMG-Guard-/15-713060/?page=3
Unfortunately, on the old archived thread, it may be hard to figure out which posts were made by CMMG.
That thread is 25 pages long but a lot of good info in there.

Here is a screenshot I saved on my website of a post by 'CMMG' regarding the angles which BTW, they never did.  It is understandable.....much easier to just have customers add/remove weights from the carrier than have so many different bolt variations.
However tuning via different angles is better IMHO vs adding more reciprocating mass.



Link Posted: 6/16/2023 12:33:35 PM EST
[#14]
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By SSTPAC:
CJofFL what all did you receive for the replacement bolt? Was it the complete bcg repair kit or just a replacement bolt only?
View Quote


Only an assembled bolt.
Link Posted: 6/23/2023 12:06:44 PM EST
[#15]
So I received my replacement bolt in the mail today, right off the bat there are visual differences. The bolt is marked "50" on the bottom of the bolt, right behind the skinny lugs. The lug angle is much shallower and I assume that the "50" marking refers to lug angle. My original bolt has no numerical markings on it and the lug angle is much steeper. The firing pin spring on my original bolt is 1-1/8" long and 8 coils, the spring on the replacement bolt is 1-1/2" long and is much tighter wound, 18 coils. The extractor on the original bolt had a spring and a small rubber bumper, no o-ring. The replacement bolt has a much larger rubber bumper, spring and o-ring. The extractor itself is more dehorned and doesn't have the sharp corner edges. Overall machining is much better, very few machining marks, very smooth surface. The nitride is very smooth also. overall a very nice part. Have not fired yet just reassembled with new bolt.
Link Posted: 6/23/2023 4:11:56 PM EST
[#16]
I wonder if this updated bolt is why my girlfriend's factory CMMG Banshee with all of their upgraded controls and the new handguard and finish seems to have less recoil than mine which was purchased a year earlier as just an upper?   I may have to shoot them side by side and swap BCG's around.
Link Posted: 6/24/2023 8:26:59 AM EST
[#17]
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By SSTPAC:
So I received my replacement bolt in the mail today, right off the bat there are visual differences. The bolt is marked "50" on the bottom of the bolt, right behind the skinny lugs. The lug angle is much shallower and I assume that the "50" marking refers to lug angle. My original bolt has no numerical markings on it and the lug angle is much steeper. The firing pin spring on my original bolt is 1-1/8" long and 8 coils, the spring on the replacement bolt is 1-1/2" long and is much tighter wound, 18 coils. The extractor on the original bolt had a spring and a small rubber bumper, no o-ring. The replacement bolt has a much larger rubber bumper, spring and o-ring. The extractor itself is more dehorned and doesn't have the sharp corner edges. Overall machining is much better, very few machining marks, very smooth surface. The nitride is very smooth also. overall a very nice part. Have not fired yet just reassembled with new bolt.
View Quote


I haven’t disassembled mine, but I looked at them again. The 2 on the left are from 7/22 and 10/22 (1 directly from CMMG and in response to the spring issue, so presumably current at the time) and are new. The one on the right is, I’m guessing, 2019/2020 and used. I’ll be the first to admit my eyes are bad, but there’s no numbers that I can see on any of them. I do notice now the diameter difference in the 2 parts of the bodies on the left. The angle of the lugs looks the same to me. @SSTPAC got any pics?



Link Posted: 6/24/2023 3:58:15 PM EST
[#18]
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By CJofFL:


I haven't disassembled mine, but I looked at them again. The 2 on the left are from 7/22 and 10/22 (1 directly from CMMG and in response to the spring issue, so presumably current at the time) and are new. The one on the right is, I'm guessing, 2019/2020 and used. I'll be the first to admit my eyes are bad, but there's no numbers that I can see on any of them. I do notice now the diameter difference in the 2 parts of the bodies on the left. The angle of the lugs looks the same to me. @SSTPAC got any pics?

https://i.imgur.com/HOg37zB.jpg

https://i.imgur.com/wIwPAr9.jpg
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Originally Posted By CJofFL:
Originally Posted By SSTPAC:
So I received my replacement bolt in the mail today, right off the bat there are visual differences. The bolt is marked "50" on the bottom of the bolt, right behind the skinny lugs. The lug angle is much shallower and I assume that the "50" marking refers to lug angle. My original bolt has no numerical markings on it and the lug angle is much steeper. The firing pin spring on my original bolt is 1-1/8" long and 8 coils, the spring on the replacement bolt is 1-1/2" long and is much tighter wound, 18 coils. The extractor on the original bolt had a spring and a small rubber bumper, no o-ring. The replacement bolt has a much larger rubber bumper, spring and o-ring. The extractor itself is more dehorned and doesn't have the sharp corner edges. Overall machining is much better, very few machining marks, very smooth surface. The nitride is very smooth also. overall a very nice part. Have not fired yet just reassembled with new bolt.


I haven't disassembled mine, but I looked at them again. The 2 on the left are from 7/22 and 10/22 (1 directly from CMMG and in response to the spring issue, so presumably current at the time) and are new. The one on the right is, I'm guessing, 2019/2020 and used. I'll be the first to admit my eyes are bad, but there's no numbers that I can see on any of them. I do notice now the diameter difference in the 2 parts of the bodies on the left. The angle of the lugs looks the same to me. @SSTPAC got any pics?

https://i.imgur.com/HOg37zB.jpg

https://i.imgur.com/wIwPAr9.jpg
Interesting...
So the only bolts I've seen marked are the 40SW and 10mm.
.40SW are engraved 60 and 10mm is 67.
I have a CMMG RDB in .45ACP as well but don't think it is marked since it is obvious it is .45 w/ the large bolt face.
Makes sense to mark the 40SW and 10mm since they share the same case head and make identifying easy.
Link to my webpage: https://c3junkie.com/?page_id=854






I've emailed CMMG to see if can get one of my 9mm bolts swapped out for testing.
I've guessed the numbers represent the number of degree's off the bore's axis but I never bothered to actually try to measure it.
Link Posted: 6/25/2023 8:05:26 AM EST
[Last Edit: CJofFL] [#19]
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By amphibian:
I've guessed the numbers represent the number of degree's off the bore's axis but I never bothered to actually try to measure it.
View Quote


I think you’re right, makes sense with the angle increasing with the cartridge energy. I used a 10x pocket optical comparator to measure various lugs on all 3 of my 9mm bolts and all are 50 degrees off the bore axis.

To be specific, if anyone cares, I set the corner of the lug in the corner of the reticle, lined up 90 with the long side of the lug (0 at the backs of the lugs), and they measured 40. 90 minus 40 equals 50 off bore axis.

While I was at it, I used a micrometer to measure the length of the lugs opposite the extractor and adjacent to it (not the feed lug) on all 3 of my 9mm bolts too. I didn’t expect anything by measuring it, just looking for an obvious difference between the new and old bolts. The old, used one which I did NOT clean was .279 and .279. One new one was .277 and .276 and the other new one was .276 and .276. For reference, the spec on an AR15 bolt is .278 +/-.002. I didn’t expect these measurements at all because it looks like the angle cut on the lugs of the 9mm bolts has some of the corner missing, but apparently not.

ETA: like I previously posted CMMG claimed ‘a slight change in the angle on the lugs’ as one of the differences on the new bolt they sent me. So, the new bolt doesn’t look very promising so far for fixing my spring issue considering one of the claimed differences doesn’t seem to be there.
Link Posted: 6/27/2023 4:38:18 AM EST
[#20]
" />

" />

This is what I received.
Link Posted: 7/5/2023 4:07:30 PM EST
[#21]
Did they charge you guys for the new bolt?
Link Posted: 7/5/2023 4:14:02 PM EST
[#22]
Link Posted: 7/5/2023 4:51:27 PM EST
[#23]
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By CMMGMod:

It would be covered under our Lifetime Quality Guarantee if having ejection/extraction issues and customer service deems a new bolt is necessary.
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Originally Posted By CMMGMod:
Originally Posted By Jmacken37:
Did they charge you guys for the new bolt?

It would be covered under our Lifetime Quality Guarantee if having ejection/extraction issues and customer service deems a new bolt is necessary.
I was told by support I have one coming but haven't received it yet.  Looking forward to it.
Link Posted: 7/5/2023 8:10:43 PM EST
[#24]
No I wasn't charged and the process was super simple, it wasn't a game of 20 questions either. Just told them I had problems he asked date of purchase and said I needed a new one and got my address and I got it in the mail about a week later. I did not receive any shipping information and the small bubble mailer I received did not have CMMG specifically listed as a return address just a  individuals name and address from Missouri so i figured who it was from.
Link Posted: 7/13/2023 1:19:41 PM EST
[#25]
Originally Posted By SSTPAC:
The extractor itself is more dehorned and doesn't have the sharp corner edges.
View Quote

By dehorned, do you mean anything like these pics, perhaps less extreme, but in the same spots?
Link Posted: 7/14/2023 9:23:53 PM EST
[#26]
Yes, same areas, not nearly as extreme just rounded over corners without sharp edges, and its all nitrided.
Link Posted: 7/16/2023 10:26:02 AM EST
[Last Edit: 1168RGR] [#27]
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By SSTPAC:
Yes, same areas, not nearly as extreme just rounded over corners without sharp edges, and its all nitrided.
View Quote

Would you mind replying back after some time with it? I’ve long suspected that the ejection problem is actually due to the M16 extractor, and any problem with the ejector spring was just exacerbated by that. It never made sense to me why ejector springs wearing out were so widely reported, when they last damn near forever in a 5.56 gun. Even in CMMGs, you only hear about it with the RDB guns, even though the case is lighter than a 5.56, and bolt velocity isn’t exceptionally high.

The above pics are how I solved ejection problems on straight blowback guns using light subsonic handloads. I initially thought the guns had ejector problems, but found that the extractor was holding onto the cases too much at the edges of the extractor, compared to a Colt bolt, which has a relatively narrow extractor. So I modified a few, using advanced Bubba technology, along with a file set and Cratex, with cold blue. Fixed. I’ve got a little over 15k rounds on the modified ones. 9mm rim dimensions are not the same as 5.56, and they don’t contact a M16 extractor exactly the same. A reloader can see this in shellholders.
Link Posted: 7/16/2023 1:26:50 PM EST
[#28]
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By 1168RGR:


Would you mind replying back after some time with it? I’ve long suspected that the ejection problem is actually due to the M16 extractor, and any problem with the ejector spring was just exacerbated by that. It never made sense to me why ejector springs wearing out were so widely reported, when they last damn near forever in a 5.56 gun. Even in CMMGs, you only hear about it with the RDB guns, even though the case is lighter than a 5.56, and bolt velocity isn’t exceptionally high.
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Originally Posted By 1168RGR:
Originally Posted By SSTPAC:  Yes, same areas, not nearly as extreme just rounded over corners without sharp edges, and its all nitrided.


Would you mind replying back after some time with it? I’ve long suspected that the ejection problem is actually due to the M16 extractor, and any problem with the ejector spring was just exacerbated by that. It never made sense to me why ejector springs wearing out were so widely reported, when they last damn near forever in a 5.56 gun. Even in CMMGs, you only hear about it with the RDB guns, even though the case is lighter than a 5.56, and bolt velocity isn’t exceptionally high.


What I think is killing the ejector springs in the CMMG, and why they went to an ejector not mounted on the bolt on their new action, is the case is pushing on the ejector for longer than a 5.56x45 round, that has to be pulled out of the chamber by the extractor.  Bolt thrust in the 5.56x45mm is momentary, while in a delayed blowback action like the CMMG, bolt thrust is what operates the action.
Link Posted: 7/16/2023 2:13:53 PM EST
[Last Edit: amphibian] [#29]
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By 1168RGR:
It never made sense to me why ejector springs wearing out were so widely reported, when they last damn near forever in a 5.56 gun. Even in CMMGs, you only hear about it with the RDB guns, even though the case is lighter than a 5.56, and bolt velocity isn't exceptionally high.
View Quote
I've documented this extensively on my site and still believe the root cause is the excessive headspacing which I've seen first hand in the (3) 9mm CMMG barrels I have and several others that I've handled that belonged to friends.  The .40SW, 10mm and .45ACP all have excessive headspacing which I believe is inherent in the design.  

As mentioned on my site, I had a custom barrel made with the headspacing slop removed but then the recoil impulse was so soft that it would no longer had enough bolt velocity to reliably kick the brass out.

This is what I'm talking about:


An MP5 doesn't do that.  You shouldn't see any movement with a round in the chamber and push on it like that.  

Eventhough the .45ACP CMMG also has considerable slop I haven't worn out an ejector spring in .45ACP nor have I heard of reports of that either.
9mm has higher case pressure than .45ACP.

My early CMMG RDB barrel had extremely excessive headspacing and was eating parts as I documented here:
CMMG RDB Excessive Headspace


After showing CMMG that video they swapped the barrel for me and then it wouldn't extract brass without an extractor installed and no suppressor mounted like my other CMMG factory barrel in 9mm and .40SW.

I think it is hard to get them headspaced just right since angled lugs are interfacing with a standard squared off extension lugs which is what they use.  
I know their patent references angled lugs in the extension but that is not how they are actually made/sold.

Link Posted: 7/16/2023 3:07:54 PM EST
[#30]
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By amphibian:

I think it is hard to get them headspaced just right since angled lugs are interfacing with a standard squared off extension lugs which is what they use.  
I know their patent references angled lugs in the extension but that is not how they are actually made/sold.

View Quote

To be somewhat pedantic and perhaps overly technical, depending on the reference used, the CMMG Rotary Delay Blowback system extension lugs are at an angle - a 0 degree angle or a 90 degree angle.

MHO, YMMV, etc.  Be well.
Link Posted: 7/16/2023 3:31:31 PM EST
[#31]
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By SecondAmend:

To be somewhat pedantic and perhaps overly technical, depending on the reference used, the CMMG Rotary Delay Blowback system extension lugs are at an angle - a 0 degree angle or a 90 degree angle.

MHO, YMMV, etc.  Be well.
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Originally Posted By SecondAmend:
Originally Posted By amphibian:

I think it is hard to get them headspaced just right since angled lugs are interfacing with a standard squared off extension lugs which is what they use.  
I know their patent references angled lugs in the extension but that is not how they are actually made/sold.


To be somewhat pedantic and perhaps overly technical, depending on the reference used, the CMMG Rotary Delay Blowback system extension lugs are at an angle - a 0 degree angle or a 90 degree angle.

MHO, YMMV, etc.  Be well.
I think your comments are too confusing....I  already stated they are using a standard squared off extension.

Link Posted: 7/16/2023 3:46:32 PM EST
[#32]
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By amphibian:
I've documented this extensively on my site and still believe the root cause is the excessive headspacing which I've seen first hand in the (3) 9mm CMMG barrels I have and several others that I've handled that belonged to friends.  The .40SW, 10mm and .45ACP all have excessive headspacing which I believe is inherent in the design.  

As mentioned on my site, I had a custom barrel made with the headspacing slop removed but then the recoil impulse was so soft that it would no longer had enough bolt velocity to reliably kick the brass out.

This is what I'm talking about:
https://www.youtube.com/watch?v=xhm4omLtW2s

An MP5 doesn't do that.  You shouldn't see any movement with a round in the chamber and push on it like that.  

Eventhough the .45ACP CMMG also has considerable slop I haven't worn out an ejector spring in .45ACP nor have I heard of reports of that either.
9mm has higher case pressure than .45ACP.

My early CMMG RDB barrel had extremely excessive headspacing and was eating parts as I documented here:
https://www.youtube.com/watch?v=Y0DXCouLsIw

After showing CMMG that video they swapped the barrel for me and then it wouldn't extract brass without an extractor installed and no suppressor mounted like my other CMMG factory barrel in 9mm and .40SW.

I think it is hard to get them headspaced just right since angled lugs are interfacing with a standard squared off extension lugs which is what they use.  
I know their patent references angled lugs in the extension but that is not how they are actually made/sold.

View Quote View All Quotes
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Originally Posted By amphibian:
Originally Posted By 1168RGR:
It never made sense to me why ejector springs wearing out were so widely reported, when they last damn near forever in a 5.56 gun. Even in CMMGs, you only hear about it with the RDB guns, even though the case is lighter than a 5.56, and bolt velocity isn't exceptionally high.
I've documented this extensively on my site and still believe the root cause is the excessive headspacing which I've seen first hand in the (3) 9mm CMMG barrels I have and several others that I've handled that belonged to friends.  The .40SW, 10mm and .45ACP all have excessive headspacing which I believe is inherent in the design.  

As mentioned on my site, I had a custom barrel made with the headspacing slop removed but then the recoil impulse was so soft that it would no longer had enough bolt velocity to reliably kick the brass out.

This is what I'm talking about:
https://www.youtube.com/watch?v=xhm4omLtW2s

An MP5 doesn't do that.  You shouldn't see any movement with a round in the chamber and push on it like that.  

Eventhough the .45ACP CMMG also has considerable slop I haven't worn out an ejector spring in .45ACP nor have I heard of reports of that either.
9mm has higher case pressure than .45ACP.

My early CMMG RDB barrel had extremely excessive headspacing and was eating parts as I documented here:
https://www.youtube.com/watch?v=Y0DXCouLsIw

After showing CMMG that video they swapped the barrel for me and then it wouldn't extract brass without an extractor installed and no suppressor mounted like my other CMMG factory barrel in 9mm and .40SW.

I think it is hard to get them headspaced just right since angled lugs are interfacing with a standard squared off extension lugs which is what they use.  
I know their patent references angled lugs in the extension but that is not how they are actually made/sold.


Interesting theory that makes a lot of sense.  Mine really carbons up the brass with everything but NATO 124 factory loads.  Most brass ejects looking like it was fired from a suppressed gun, very black soot all over the outside.  Mine is an earlier production 9mm that ran flawlessly for 1k rounds or so, then suddenly seemed to just quit ejecting about half the time.  I bought a new set of springs and a complete new bolt assembly.  I replaced all the springs on the old bolt and it still wouldn't function reliably, so I installed the new bolt assembly that looked identical at a glance and it has ran fine since, although I have put a hundred or so rounds through it since.
Link Posted: 7/17/2023 7:18:30 AM EST
[#33]
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By amphibian:
I've documented this extensively on my site and still believe the root cause is the excessive headspacing which I've seen first hand in the (3) 9mm CMMG barrels I have and several others that I've handled that belonged to friends.  The .40SW, 10mm and .45ACP all have excessive headspacing which I believe is inherent in the design.  

As mentioned on my site, I had a custom barrel made with the headspacing slop removed but then the recoil impulse was so soft that it would no longer had enough bolt velocity to reliably kick the brass out.

This is what I'm talking about:
https://www.youtube.com/watch?v=xhm4omLtW2s

An MP5 doesn't do that.  You shouldn't see any movement with a round in the chamber and push on it like that.  

Eventhough the .45ACP CMMG also has considerable slop I haven't worn out an ejector spring in .45ACP nor have I heard of reports of that either.
9mm has higher case pressure than .45ACP.

My early CMMG RDB barrel had extremely excessive headspacing and was eating parts as I documented here:
https://www.youtube.com/watch?v=Y0DXCouLsIw

After showing CMMG that video they swapped the barrel for me and then it wouldn't extract brass without an extractor installed and no suppressor mounted like my other CMMG factory barrel in 9mm and .40SW.

I think it is hard to get them headspaced just right since angled lugs are interfacing with a standard squared off extension lugs which is what they use.  
I know their patent references angled lugs in the extension but that is not how they are actually made/sold.

View Quote
yes, I’ve read your site and what you’ve written here. Its a good read.

I’m convinced that the slop is a problem, and maybe inherent, but not convinced its killing the ejector springs. Perhaps, but I’m not sure.

.45 ACP isn’t a good comparison for what I’m talking about, which revolves around rim dimensions. Pressure isn’t relevant to my point, and wouldn’t make sense anyway, since 9mm is lower pressure than the 5.56 that the bolt face is designed around.

Bolt thrust powering the bolt in a blowback is exactly why my extractor mod works on a straight blowback gun. I don’t really know if that would work on a CMMG RDB or not, though. But in a regular blowback, a wide extractor is unnecessary, and Colt chose to design a bolt with a very narrow one. They could have used the M16 extractor but chose not to, for whatever reason that may be. In the CMMG, the bolt rotates, so I could very well be wrong. But I suspect its holding the shell in a less than optimal way, and that’s why they chamfered those edges in the latest version.

They seem to have eventually reached the same conclusion I did.
Link Posted: 7/17/2023 7:57:10 AM EST
[Last Edit: amphibian] [#34]
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By 1168RGR:
yes, I've read your site and what you've written here. Its a good read.

I'm convinced that the slop is a problem, and maybe inherent, but not convinced its killing the ejector springs. Perhaps, but I'm not sure.

.45 ACP isn't a good comparison for what I'm talking about, which revolves around rim dimensions. Pressure isn't relevant to my point, and wouldn't make sense anyway, since 9mm is lower pressure than the 5.56 that the bolt face is designed around.

Bolt thrust powering the bolt in a blowback is exactly why my extractor mod works on a straight blowback gun. I don't really know if that would work on a CMMG RDB or not, though. But in a regular blowback, a wide extractor is unnecessary, and Colt chose to design a bolt with a very narrow one. They could have used the M16 extractor but chose not to, for whatever reason that may be. In the CMMG, the bolt rotates, so I could very well be wrong. But I suspect its holding the shell in a less than optimal way, and that's why they chamfered those edges in the latest version.

They seem to have eventually reached the same conclusion I did.
View Quote
My comment regarding pressure was comparing 9mm to .45ACP since the .45ACP RDB doesn't have the issue while the higher pressure 9/40/10mm all have the problem.
Comparing pressures to 556 is also irrelevant since the bolt is truly locked not delayed.
As mentioned on my site when suppressed, the CMMGs will extract with no extractor even installed but the ejector spring gets mangled in a as little as 1 magazine.

ETA.  I do think the extractor having less surface area will help extend the amount of rounds you can run before having to swap ejector springs.
A new Mk10 spring is like 10lbs to compress.  Once it gets under 5lbs it will start having ejection failures but may keep running with these extractor mods....but at some point when the spring gets too weak it will have ejection failures.
Link Posted: 7/19/2023 1:10:55 AM EST
[#35]
I put the MK10 spring in my girlfriends CMMG 9mm probably less than 400 rounds ago and it is already having FTE problems. I was hoping this would be a more long term solution than that.
Link Posted: 7/19/2023 1:28:38 PM EST
[#36]
I received the new bolt on Monday.
I can't see a difference in the extractor lip.









Link Posted: 8/6/2023 2:37:02 PM EST
[#37]
Did those of you with new bolts note any improvement?
Link Posted: 8/7/2023 12:01:56 PM EST
[#38]
Discussion ForumsJump to Quoted PostQuote History


Thanks. On your old one, that’s not wear I see at the edges, is it? I think its just the camera angle, but you seem to shoot a lot of full auto, so I thought I’d ask.
Link Posted: 8/7/2023 1:12:48 PM EST
[#39]
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By 1168RGR:


Thanks. On your old one, that's not wear I see at the edges, is it? I think its just the camera angle, but you seem to shoot a lot of full auto, so I thought I'd ask.
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Discussion ForumsJump to Quoted PostQuote History
Originally Posted By 1168RGR:


Thanks. On your old one, that's not wear I see at the edges, is it? I think its just the camera angle, but you seem to shoot a lot of full auto, so I thought I'd ask.
Yes, there is some wear but it seems to just wear to a point and stop....don't see them getting worse.
Link Posted: 8/7/2023 3:22:46 PM EST
[Last Edit: Jmacken37] [#40]
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By Jmacken37:
Did they charge you guys for the new bolt?
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Discussion ForumsJump to Quoted PostQuote History
Originally Posted By Jmacken37:
Did they charge you guys for the new bolt?

Originally Posted By Jmacken37:
Did they charge you guys for the new bolt?


CMMG customer service just told me to pound sand (in so many words) with the below response from Andrew, who encouraged me to have a "blessed day," because I didn't purchase a complete rifle.

Thanks CMMG! I will remember this experience when I decide to make future purchases.

Hi Jaco!


This means that with these components installed in a non CMMG platform (which these parts were designed for) that i cannot guarantee functionality or reliability with that configuration of parts. I can guarantee functionality and reliability with these parts if they are built according to our factory callouts using our parts. Thank you for your understanding, and please let me know if you have any further questions.



Have a blessed day,


Andrew
CMMG Customer Service
Link Posted: 8/7/2023 5:44:33 PM EST
[#41]
I still look at how few you see in steel challenge or USPSC pcc. If they were reliable you would see more of them. JP5, sig Mpx and plain old blowback are still the guns that win.
Link Posted: 8/8/2023 8:48:47 AM EST
[Last Edit: Jmacken37] [#42]
Well, to their credit, another CMMG employee, Collin, reached back out and offered to send me one of these newer bolts. Here's hoping it fixes my issues.
Link Posted: 9/3/2023 9:44:50 AM EST
[#43]
Just about over it with my CMMG RDB build. New Mk10 spring is not working and have no where near the shot count that most are aaying their issues start. Just don't feel like breaking down and cleaning the bolt every 300 rds and replacing springs. If headspace is the issue and they sell barrels separate, will getting a new barrel have a better headspace than my current one. I can see them not honoring the lifetime guarantee as there are tons of bolts and barrels out there that will start eating up profits quick if they send out a bunch of free parts. Going to pull it all apart one last time and check for headspace issue then off to hunt for a grand power stribog A3. Would love to keep my CMMG setup suppressed if it would run but tired of fighting it. May just need to convert it to blowback and salvage what I can.
Link Posted: 9/4/2023 2:23:54 PM EST
[#44]
Well started parts gathering for a PSA AR-V and Bowden MP5 handguard. Lookjng to build an MP5 copy with Martrix Roller Delayed buffer system. Really want to do the PDW short stock but I think I will just rock a Magpul Fixed Carbine stock to save a little money. Will be 100% suppressed with 8" barrel. We shall see if this runs better than the CMMG RDB junk.
Link Posted: 9/4/2023 4:25:42 PM EST
[Last Edit: amphibian] [#45]
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By Joedirt199:
Well started parts gathering for a PSA AR-V and Bowden MP5 handguard. Lookjng to build an MP5 copy with Martrix Roller Delayed buffer system. Really want to do the PDW short stock but I think I will just rock a Magpul Fixed Carbine stock to save a little money. Will be 100% suppressed with 8" barrel. We shall see if this runs better than the CMMG RDB junk.
View Quote
Uh...did you see that I posted that I was very disappointed with the Maxim Defense RDB system suppressed.  Myself and CJofFL met up at the range and we both thought it was LOUD.
https://www.ar15.com/forums/AR-15/Maxim-Defense-RDB-review/15-780933/?page=6
The same suppressor used on my CMMG RDB or my MP5 was very quiet.

I would not get a Maxim Defense setup for use with a suppressed configuration unless it is with the belt fed FM-9.  It is quiet with that configuration.

Link Posted: 9/4/2023 5:19:20 PM EST
[#46]
Can't do much shooting when the CMMG just jams all the time. Still don't see where people are having any luck with the new bolt they are sending out and if CMMG is going to pick and choose which customers get warranty support or not, then I guess it is time to move in another direction.
Link Posted: 9/7/2023 8:00:02 PM EST
[#47]
I started this thread because I had bought a bolt and barrel combo in January of this year for a pistol assembly and after a 100 round break in it was shooting well so I was thinking of an RDB rifle but wanted to know the latest on whether or not the Mk10 spring solved the ejection issues. Based on the responses in this thread the answer is definitely no, but I did learn that CMMG appeared to have stopped offering the Mk10 spring as a fix and was instead offering a revised bolt assembly for ejection (and extraction) complaints. This bolt assembly is marked "50" as previously mentioned. I checked the bolt that was included in the combo that I bought and it was marked "50" so I knew that it was not only being offered as a fix but was being used in production.

I am now at that infamous 1500 round or so mark where failures have been reported but I think requesting the revised "50" marked bolt is a plus if you are having extraction/ejection issues. The pistol has been performing well in terms of function. I am using endo mags and a 300 BO flat wire spring. The only issue for me now is poor accuracy which has been since the beginning. I have tried 15 or so different brands/types within brands of ball ammo and cannot get better than a fist sized pattern at 25 yds except for 3 brands and some JHP ammo I've tried.

I'd like to do a rifle but I'm thinking I don't want to be tied to their barrel although I know accuracy is not exclusively a barrel issue.
Link Posted: 9/7/2023 8:29:09 PM EST
[Last Edit: backbencher] [#48]
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By 9nzz:
I started this thread because I had bought a bolt and barrel combo in January of this year for a pistol assembly and after a 100 round break in it was shooting well so I was thinking of an RDB rifle but wanted to know the latest on whether or not the Mk10 spring solved the ejection issues. Based on the responses in this thread the answer is definitely no, but I did learn that CMMG appeared to have stopped offering the Mk10 spring as a fix and was instead offering a revised bolt assembly for ejection (and extraction) complaints. This bolt assembly is marked "50" as previously mentioned. I checked the bolt that was included in the combo that I bought and it was marked "50" so I knew that it was not only being offered as a fix but was being used in production.

I am now at that infamous 1500 round or so mark where failures have been reported but I think requesting the revised "50" marked bolt is a plus if you are having extraction/ejection issues. The pistol has been performing well in terms of function. I am using endo mags and a 300 BO flat wire spring. The only issue for me now is poor accuracy which has been since the beginning. I have tried 15 or so different brands/types within brands of ball ammo and cannot get better than a fist sized pattern at 25 yds except for 3 brands and some JHP ammo I've tried.

I'd like to do a rifle but I'm thinking I don't want to be tied to their barrel although I know accuracy is not exclusively a barrel issue.
View Quote


Have you checked headspace?.  Amphibian has done all sorts of things w/ his CMMGs, but one of his consistent complaints has been a huge chamber.
Link Posted: 9/7/2023 10:21:43 PM EST
[#49]
Link Posted: 9/8/2023 6:20:35 AM EST
[#50]
I had good luck with Rocky Mountain Reloadings 124 gr Matchwinner bullet in my 8" barrel. I was doing some testing with different powders and they seemed to hover around 1.5" groups at 25 yards.
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