CMMG upper issues

[civilian75]

Recently acquired this CMMG upper. On my first outing I had the following failures in 20 shots fired:
* 100% failure to peel off next round from mag. tried two diff mags (USGI & Magpul)
* ~40% failure to eject.
* two bullets key holed
* larger than expected vertical stringing.

Background:
* Used an unmodified M4 lower (milspec buffer & recoil spring).
* YHM308 suppressor attached at all times.
* did not try firing w/o suppressor. Did not see the point.
* Fired handloads, Hornady 225gr Match on top of 10.5gr A1680, OAL: 2.160", crimped with Lee Factory crimp die
* Clocked a few of the shorts: n:11; Hi: 1053fps; Lo: 977fps (cold bore), 1012fps(hot bore); x:1026fps; Stdev: 21

I admit my handloads may need some improvements. I will try increasing the charge and OAL while keeping velocities just over 1010fps. But, despite my lack of experience with the AR platform, to me, it all seems to point to a gas port that is not large enough. Before I go off messing around with my loads, maybe someone out there can suggest something less obvious I should look at. For now, these are the things that come to mind:
* somehow the load is too weak and I need to figure how to increase chamber pressure w/o increasing muzzle velocity
* need lighter buffer and/or weaker recoil springs
* The new upper gas port hole somehow is either a) blocked/dirty, or, b) diameter too small from factory.

By the way, I am not interested full power 110-125gr ammo, just subsonic work. Should I regret getting a carbine length barrel?

Thanks
Civvy

[civilian75]

I could not wait for a reply. I took out the gas block. Port is 0.110". Don't feel comfortable drilling a larger hole. So, it is now either weaker spring or somehow a hotter load (?)

[SpaceWrangler]

That's quite a list of seemingly unrelated problems for a new Upper.

A misaligned gas block may explain the gas issue.

1/8 should be plenty of twist to stabilize a 225.

Your Extractor should have nice sharp edges and also have the five-coil spring and black insert.

It's possible the barrel isn't tight. Hold the Upper Receiver tight and see if your barrel twists slightly in the Receiver.


Contacting CMMG would probably be the best thing.

[Notar]

I just sold my CMMG Carbine gas 16 inch 300BLK upper. It was a Great shooting upper with supersonic loads. On occasion the bolt would not lock to the rear when I wasn't holding the rifle really tight to my shoulder. Normal stance and bolt open worked fine. I took one of the weights out of a standard carbine buffer, and the bolt always would lock open on an empty mag.

I did fire a box of Rem 220 subs, they shot just over MOA with no tumbling of the bullets. My bolt had the black insert under the extractor, it took some pressure to get the extractor back in place.

I've never had to send anything back to CMMG, but I've worked a project with them, and they were top notch to work with, and I hear the customer service is good. They even answer the phone!!!!!!!!!!!

Let us know what it takes to get you running 100%

Pistol gas works pretty good with subs.

[civilian75]

Correction: ~40% fail to eject. (sorry, my brain sometimes goes dyslexic with both terms). I have 0% extractor issues. My bad.

[SpaceWrangler]

Okay, that evens things out a bit. Short stroking (incomplete cycling) can easily contribute to Failure to Eject... the Bolt Carrier isn't going back far enough to eject the spent case before the carrier comes back forward.

And in not going all the way to the rear, the carrier can't strip a fresh round from the top of the magazine.

Since you've already removed the Gas Block... from looking at the carbon marks on the Gas Block and around the Gas Port, did the block seem to be properly aligned with the Gas Port?

Before I go off messing around with my loads, maybe someone out there can suggest something less obvious I should look at. For now, these are the things that come to mind:
* somehow the load is too weak and I need to figure how to increase chamber pressure w/o increasing muzzle velocity
* need lighter buffer and/or weaker recoil springs
* The new upper gas port hole somehow is either a) blocked/dirty, or, b) diameter too small from factory.

An AR isn't like a 1911 in that you can tune your gun to your load by using different strength recoil springs. The springs come in two variations, carbine length or rifle length. If a spring isn't too short from being worn out, it should be good to go.

AR's can be tuned with lighter or heavier buffers. The lightest is the unmarked Carbine buffer, then H, H2, H3 as they get heavier.

If your gas block is aligned properly and your gun has an unmarked (lightest) buffer, about all that it could be is your loads.

Try testing your gun with proven ammo, such as the Remington 220 gr. subsonics mentioned by Notar above.

[civilian75]

This is good feedback. For the life of me I could not describe the failure as "short stroking", the perfect term to describe exactly what's going on.

I had previously tested this ammo with another 300 blackout upper, same unmodified M4 lower, and the bolt locked back on the last shot. I believe it is the best indication it wasn't short stroking.

It doesn't seem I fired enough shots to create noticeable carbon fouling. Or maybe, since I wasn't looking for it, I did not see it. I measured holes alignment and they seemed fine. Yet, I admit, it was in less-than-optimal conditions and I may have incurred in some error. The question begging is, if they are misaligned, what next, drill a bigger hole on the gas block? If they are not misaligned, is a 0.110" hole large enough or should I go for 0.120"?

I took a 280gr weight out of the "H" buffer, re-oiled and cleaned everything. Don't know when I'll have a chance to fire test the rig again, though, maybe tonight. Ordered online a "reduced power" Wolff recoil spring just in case I need it.

[SpaceWrangler]

Try factory ammo first. You've already expressed doubt about your handloads.

I would return the Upper to CMMG for evaluation before I started drilling bigger holes or anything else that would void the warranty.

[civilian75]

I would return the Upper to CMMG for evaluation before I started drilling bigger holes or anything else that would void the warranty.

Warranty is already voided. here is what CMMG said:

...as for the warranty, since you have used re-loaded ammunition (Hornady 225gr Match, 10.5gr A1680, OAL:2.160", clocking 1020fps
average) , it has already been voided: http://www.cmmginc.com/pages/ammunition.html

[civilian75]

Finally, some light at the end of the tunnel!

First, what did not work: lightening the buffer. CMMG's technical support suggested to replace the H bugger with a lighter one. I removed the 34g weight and replaced it with a wood dowel of the same dimensions, left the two 18gr weights put, but were identical: more short stroking. Bottom line, the standard solution, changing to a lighter buffer, may not work after all.

What is showing promise is replacing the BCG. I have a 16"bbl carbine upper with a fully broken-in upper receiver and BCG. Swapped BCGs and voila! 100% full strokes. Bolt would peel the next round, and locks back on empty magazine. Then, I borrowed a BGC from a friend's new 300 BKO upper that looks identical to my old BCG, but it short strokes, too. In summary, it seems that both BCG and upper receiver need to be broken in.

A Wolff reduced power spring is already on its way, which I am sure will help get more reliable.

Any suggestions, besides making the swap permanent?