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CMMG Observations


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I have both my 8" CMMG uppers running well with my two AR-15 lowers.  I've gotten good support from CMMG working through ejection issues seemingly related to some outdated spring configurations in the bolt.  I'm running a standard buffer configuration.

  • If you use the tan PMags don't load them to 30 rounds.  The spring pressure is sufficient to spread the lips of the mag body.  I can't insert full tan mags in my ADM lower.  Black mags are fine.
  • Don't load mags on a closed bolt as a standard practice.  The bolt lugs will score a coated bullet and scratch FMJ.  This does not occur during live fire.  The slightly damage round will cycle just fine, however.
  • The CMMG Mk10 ejector spring and a standard (not extra power) extractor spring are key to achieving reliable extraction.  When I was trouble shooting my second upper I tried a BCM extractor spring and that was an absolute no-go.
  • I recently ran 147gr Lawman, 115gr reloads and 145gr reloads through the newer upper.  I had one FTE on the first round of a ful mag.  I attribute this to the high pressure on those first two rounds.  I had no other issues and the following 28 rounds of the 115gr mag cycled just fine.  I'll do some additional testing and will continue to load to 30 rounds in practice.

YMMV of course.  Different buffer configurations and ammo will alter the bolt cycle and may affect ejection.  In my situation the #2 upper is slated for suppressed home defense to the 147 Lawman is a proxy for the Federal HST HD rounds.  I'll run the 145gr ammo in Steel Challenge and USPSA; very low recoil and LRBHO seems 100%.

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Rick... thanks to a recommendation by Memphis Mechanic, the BCM extractor spring worked for me (I don't think CMMG was offering the 40SW spring as a fix, or at least I hadn't heard of it).. in my particular case, the OEM helper plug in the spring had hardened at around 1500 rounds... I've shot only a couple of matches before the Wuhan Flu cut back my participation... I use spring only, no polymer helper plug or O ring... so far the factory ejector spring is working ok... as you commented, each critter is a little different...I use 40 round ETS mags, 135 Bayou and WSF,and get a scrape on the next to present cartridge as the bolt cycles... regards

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  • 1 month later...

Yes, after replacing the ejector spring with an extra-power Wolf and (IIRC) taking the plug out of the extractor spring, mine has run 100% on both an 8" barreled upper and a 16" that I had cut back 4" but had a linear comp pinned/welded to bring it back up to 16".  Been using an Armaspec H2-equivalent captured recoil spring.  All factory ammo so far.

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Some additional information:

Did some additional testing with both my 8" barrel CMMG.  They run 100% with every type of factory ammo I had on hand from 115 gr. through 147gr including defensive rounds.  Ejection pattern very consistent between the two rifles.

 

I get the occasional FTE with hand loads ranging from 115gr through 124gr up to 145gr.  Since I don't shoot USPSA these are not factory loads and I'm now convinced that these FTE are due to the large SD I would expect from mixed headstamp cases with no control over the number of times they have been reloaded.  I do get infrequent cracked cases, but frequency is less than that of the FTE.  I'm going to bump up these loads to more closely match factory performance.  I'm am about to run out of some back of the cabinet N320 after which I'll return to Sport Pistol and introduce a new variable.

 

Yes, I know I should be using my Labradar chrono, but my I'm lazy and my 1911 eats everything and doesn't seem to care about SD, cracked cases, etc.    Next time I bring a rifle to practice I'll set up the chrono and see what I get.

Edited by RickT
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Follow up:

I upped my 125gr load from 3.8 to 4.3gr of N320 using 125gr BBI. 90 rounds later with absolute no issues and I was shocked that the rifle shot much flatter.  I could fast double on a 18x24 plate at 50 yards with virtually no correction for the second shot.  The soft load really drove the sight off the target.  I plead guilty to falling prey to the steel challenge mantra "softer is better" which does not appear to apply to my CMMG SBR.

 

I know .5 gr is a big jump, but well within the published load range and impulse is still considerably less than factory ammo.  I'm going to repeat this test with a slightly stouter 145gr load, currently only 3.2gr of N320.

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  • 2 weeks later...

I have found that the higher the power factor the flatter it shoots. I have a abundance of 124 pills to loadBut was offered some 115 grain and with some WST I was able to get 137 pf and it was just flat.. I did increase my 124gr load to 138 pf and very small difference.

 

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On 10/28/2020 at 6:25 PM, lstone said:

I have found that the higher the power factor the flatter it shoots. I have a abundance of 124 pills to loadBut was offered some 115 grain and with some WST I was able to get 137 pf and it was just flat.. I did increase my 124gr load to 138 pf and very small difference.

 

I have found this as well.  Like with other PCC's (which are all straight blowback), a faster cycle equals less dot bounce/lingering for me.  So I'll generally take a little more jolt to the shoulder over a softer kick but longer cycle and more lingering/dot movement.  The CMMG RDB helps to mitigate some of that initial kick, but I still like the faster cycling, which is nice because you don't have all the extra added mass of a 9mm bolt and buffer.

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9 hours ago, Texaspaul said:

What spring and buffer are you running in CMMG?  Mine seems to cycle slower with my loads 105gn 3.2gn Sport Pistol with SRP for Steel Challenge. 

That seems like and incredibly soft load and I'm surprised you don't see failures to eject.  I haven't switched back to SP yet, but with N320 I seem to need about 0.2gr more than SP for reliable cycling.  My 115gr N320 load would be 4.2gr using coated bullets in an 8" barrel.  My current 124gr BBI load is 4.3gr N320.  I'm a hack B shooter so YMMV, but the gun, as mentioned above, is very flat shooting with these sorts of loads.  I gave up on applying the "softer is better" logic that I use for my 1911 to the PCC.

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Had another match this morning and she's still running like a top, really liking the recoil impulse and the fact that the dot doesn't move much from target.  My controlled pairs are noticeably faster and tighter than before.  This is still with factory 115-gr FMJ.  Lower is with an Armaspec H2-equivalent captured recoil system, Hiperfire Reflex trigger, CMMG Endomag inserts for PMags.

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One final thought regarding CMMG OAL: it's not unusual to see OPs request help with bullet weight, powder load and OAL.  My 8" CMMG barrels have an extreme short leade.  I've had no issues with factory ammo, but in my experience the driving band of factory stuff does no extend beyond the case and the ogive is such that the bullet tapers pretty rapidly outside the case mouth.  I can load the old style Bayou 124gr bullets to 1.14" or more with very little of the driving band surface exterior to the case, but the BBI 125gr bullets, which function 100%, I load to 1.11".  115gr Bayou, the new profile, need to be loaded a bit shorter yet.  So, YMMV, but this has been my experience.

Edited by RickT
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  • 3 weeks later...

FYI there are 3 different Mean Arms CMMG inserts. Gen 1s are known to have problems and Mean Arms will replace Gen 1s free of charge if you are having issues. 

 

I'm running some Gen 2s, and so far only have had some cycling issues with some 135PF 147gr flat nose handloads (Carbine Buffer and standard Carbine Buffer Spring). Hotter loads have been running fine. 

 

I have some Gen 3s, but haven't had a chance to test them out yet. Looks like the only difference is the follower between Gen 2 and 3. 

 

Here some pics:

 

CqYdboBh.jpg

 

ZjtcQIhh.jpg

 

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I believe all mine are Gen 2, but I'll take a look tomorrow.  Looks like the Gen 3 follower is a mirror image?  I do know with my suspected Gen 2s if I load on a closed bolt the top round will get scratched.  Wonder if the modified follower helps with that?  I just order 3 additional inserts from Mean Arms which I assume will be Gen 3.  Interesting.

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  • 3 weeks later...

My home defense PCC now mounts an Omega 9K suppressor.  Rather than add some carrier weight I replaced the buffer on both PCC with the A5/RB5007 with Tubbs 556 buffer spring.  Every ammo I've tried has run 100% and I understand that the same buffer configuration is fine for 5.56.  Seems softer than using the stock carbine buffer, but I've already converted both lowers so I won't have a real seat-of-the-pants comparison.  Not that I need a backup to my competition PCC, not being a serious competitor, but I will use a 3-lug suppressor mount on both rifles for flexibility.

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