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ColdIron44 02-22-2012 07:57 PM

New CMMG Issues
Against my better judgement, I have been encouraged to start a thread concerning some issues I am experiencing with my new CMMG.

The gun: New-In-Box CMMG MK-4, carbine length DGI 16" 1:9 5.56 barrel, no markings on the head of the buffer leads me to guess it's a standard H?
Took it dripping wet out of the box and immediately fed it some brass.

The ammo: PMC brass 55 grain .223 FMJ

The issues: exactly 25 out of 300 rounds failed to eject, seemingly due the BCG's inability to retract completely. I have not played with any variables yet (ammo quality, .223 vs. 5.56, buffers, or lube, which I never applied in the course of 300 rounds), so the the FTE is less of a concern for me... For now.

What's eating my lunch is the trigger. The mechanics of triggers is still a bit foreign to me, and I have much to learn in this area. The following scenario occurred randomly throughout the 300 rounds fired, approximately 25% of the time:
A round was fired, the gun would cycle, but the trigger would not reset. Nudging the trigger gently forward, towards the muzzle, would set the trigger, at which point it could then be fired. My common sense, or lack thereof, leads me to believe this issue is unrelated to the FTE, and I am thus hard pressed to think of testable variables I can alter. The only thing that comes to mind is my failure to lubricate the gun.

I'm posting in this forum for the first time, but I have been following some of you gentlemen religiously for a few weeks now, and I have a tremendous amount of respect for many of you. Any input, helpful or hateful, is greatly appreciated.


mjkeat 02-22-2012 08:07 PM

I have no experience w/ this problem but would make sure the trigger/springs are installed properly and lubed, then lube the AR and give it another go.

pagj17 02-22-2012 08:11 PM

Have you contacted cmmg? My ar eats that pmc nicely, and it's ' under gassed' some. Definitely talk to them and see what they say. Sounds like qc let a bad one through.

fsted2a 02-22-2012 08:16 PM

Seems to me there was a post a while back with the same symptoms, and the shooter switched from PMC to another brand (I beleive it was Remington) and the problem was resolved. I would try MJ's suggestion first to see if it isn't a lube issue, or FCG. Then maybe a box of different ammo. My guns will shoot crap for ammo, but not every AR will.

Car54 02-22-2012 10:27 PM

I think like MJ and fsted wrote, check the springs inside the lower for the trigger reset problem. As far as the FTE's, a friend of mine had a bolt problem with his CMMG. Call their customer service, they'll help you out.

Quentin 02-22-2012 10:42 PM

Welcome to FTF, Tom!

Since your buffer is unmarked it's probably a standard carbine weight not an H buffer. If you have a postal scale, etc. and want to make sure, the standard buffer weighs 2.9oz and the H buffer weighs about 3.7.

Along with the prior advice I'd take the upper off the lower receiver and see how smooth the BCG moves back and forth in the upper. You want to especially lube where the carrier contacts the receiver. You may want to put that PMC aside and try something stronger for a hundred rounds to see if the carrier goes back far enough then.

From what you said about the trigger action that sounds normal. Try it while the upper is off but catch the hammer with your thumb, don't let it slam into the bolt catch lever. Cock the hammer, pull and hold the trigger back then cock the hammer again. The disconnector will catch the hammer then you release the trigger and the sear catches the hammer. There is a noticable click when that happens. Pull the trigger (catching the hammer with your thumb), hold the trigger, cock and release trigger. Do this over and over and make sure it always works the same. Also test the safety to make sure it works 100%. I think you're ok.

Edit: if the trigger is intermittent as you said then of course something is wrong with the fire control group. Usually an easy fix, maybe the local shop you got the AR from can take a look at that and probably fix it on the spot.

The failure to extract problem often is called shortstroking and if it continues could be caused by a gas leak. Take a look at the carrier key and make sure it's not loose and that it's staked around the two screws that hold the key to the carrier.

ColdIron44 02-23-2012 02:12 AM

I thank you all so much for your input. This is my first AR, and the malfunctions are forcing me understand its mechanics, a process I am thoroughly enjoying. If I can't iron out these issues I reckon y'all will be hearing from me again!

fsted2a 02-23-2012 01:03 PM


Originally Posted by ColdIron44 (Post 717518)
I thank you all so much for your input. This is my first AR, and the malfunctions are forcing me understand its mechanics, a process I am thoroughly enjoying. If I can't iron out these issues I reckon y'all will be hearing from me again!

If you have a buddy with an AR, you might want to see if he will let you swap upper and lower with him to see if you can narrow it down a bit. Try his upper and your lower, then his lower and your upper. I have done that before, and it saved me a lot of time trying to troubleshoot a problem. (Mine turned out to be a bad lower receiver, but I had different symptoms altogether.)

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