New CMMG MK3 w/ Issues - Ejection Issues - Overgassing Problem? Help Appreciated!

Help and advice needed please.


Well, I finally bit the proverbial bullet and picked one of these up - it's the CDNN 16" SS/Cerakote version. So far I have been less than impressed with several issues, one of which is really starting to drive me a bit nuts. I've had the rifle for 2 weeks now and it been to the range twice for a total of around 200 rounds. I'm not a big believer in "break-in" to fix major function issues - a rifle should run out of the box. It may run smoother after a while but basic functionality should be a given. Apparently I'm not the only person having this issue so I'm hoping that CMMG will jump in an offer some advice. This is my first CMMG rifle and so far it's not been a very positive experience.


I am having some failure to eject issues but I was hoping that a few more rounds work work out the bug. Unfortunately, that doesn't appear to be the case as today was the second trip out and the problem is still very much there. I've tried multiple ammunition types in hopes that if it was an overgassing issue, I could find a load that would at least let me get through a full magazine - no luck. The rifle has shown the same issue with everything I've run through it - Hirtenberger surplus, Lake City ball, PRVI 168GR Match, Pindad surplus, and Federal GMM. Everything is clean and the bolt/carrier are well lubed. The Hirt Surplus fails around 2-3 time out of 20 rounds, the PRVI and GMM about the same. The big loser is the Lake City - it fails a solid 4-5 times out of ten.













The only clue is the ejected shells - they all seem to indicate an overgassing issue. They are falling anywhere from 1:30 to 2:30 (depending on the type). I've been shooting 5.56 AR's for years (and own several) and have never experienced anything like this before so I'm looking for any advice/confirmation. This is my first .308 and I was hoping for better based on everyone else's experiences with these rifles.


What are my options? While I know that I could try sending it back to CMMG, I'd rather solve the issue myself if possible. I hate the idea of sending a brand new rifle off for an unknown length of time and quite frankly, I've heard too many horror stories about shipping rifles back and forth without ever fixing the issue. No slight to CMMG, I've heard that their customer service is solid, I would just rather avoid the hassle. I may also swap the barrel out for the SASS version anyway but I want to get it running before I sink any serious money into upgrades.


Is this a common problem with .308 AR's? Should I just go with a heavy buffer and hope for the best or would it make more sense to go with an adjustable gas block (or gas tube)? The Slash buffer isn't cheap and if I'm going to spend the money, I'd rather do it once and solve the problem. Is this even an overgassing issue at all or could it be something more serious?


Any help you could offer would be a huge help and greatly appreciated...

Thanks!



ETA:

Sorry - just found this thread:

http://www.ar15.com/forums/t_3_121/568169_CMMG_308_ejection_problem_with_pics.html

Was there ever any resolution on this one?

Am I looking at an extractor/ejector issue even with the forward brass landing pattern?


ETA V2.0

Ejector Spring seems to have plenty of tension and no binding of the ejector itself at all - nice and smooth.

Extractor/Extractor Spring are both nice and tight and grip cleanly with no slop whatsoever.

The bolt locks back on an empty mag so I don'e believe it's a short-stroking issue.

When I pull the bolt carrier assembly back all the way (using the charging handle), the bolt face is well behind the ejection port - is this an issue?

There appears to be some gas residue around the front of the gas block. Is this normal? I've not had a SS barrel previously so I've never noticed.





I'm running out of obvious things to check - anyone care to chime in?

Please?

Here is what I have learned about 308 gas guns.

Most are over gassed using a non adjustable gas block.

Does not seem to matter how much the rifle costs they all have the same over gassing problem.

Granted some people have out of the box excellent results and no problems, but many people have sold off their rifle due to over gassing.

There is a post somewhere on this board from a guy who learned the same way I did that you need to use an adjustable gas block.

Before I finally put one on I had spent money on heavier buffers, longer buffer tubes, stiffer springs, just about everything you could think of to slow down the recoil and reduce the over gassing problem.

Nothing worked, until I read his comments and bought a JP adjustable gas block.

WOW what a difference.

All my problems of over gassing were gone.

No failures to eject, no double feeds, no burnt powder on the case necks, all the brass going into a nice small pile at 3 o'clock.

I now have put adjustable gas blocks on all of my gas guns.

You can find tune each load (I hand load) for each bullet and charge.

My AR 10s run great and it only took me two years of pulling my hair out and spending money like it was being printed in my basement before I tried the adjustable gas block.

There are many types on the market, I just like the way the JP uses four screws to tighten the block.

Try one and I think your problems will be history.

Thanks much - Depending on what CMMG has to say, I may just start with an adjustable block. I don't think it can do any harm and I may just get lucky. If nothing else, it will give me some added flexibility down the road. Now I just need to decide if I plan on replacing the barrel - no sense in buying a block until I know what it's going on.

Anyone know off the top of their head what the barrel diameter is on the standard SS barrel - standard .750?

Thanks again!

Forget sending to CMMG. Had the same issues and they sent it back to me that the gun ran fine. My issues arises when shooting suppressed tho.

Here is what worked for my CMMG 308...I put 3 quarters in the buffer tube to shorten the max amount the buffer could go back, then I added a heavier buffer from heavy buffers dot com. Now my rifle runs 100% until it starts getting really dirty. My adjustable gas block made only a little difference in cycling. When adding to the buffer tube, add 1 quarter and assemble the rifle. Then pull the charging handle back and with your other hand, push back the buffer to the furthest back spot....stop adding quarters when it will only come back even with the very back of the ejection port. My buffer traveled too far, allowing spent cases to re-enter the chamber sideways, forwards, or even backwards.

Try that and let me know if anything improved.

Thanks for the suggestion - have you had any issue with buffer tube or lower damage? I have heard of people using washers (or quarters) to shorten the buffer travel but doesn't that just give it something to slam into? I'm concerned that I would be getting it to run short-term but without actually solving the underlying issue. How much slam abuse can the buffer tube handle if the spring/buffer isn't absorbing the brunt of the energy?

Which adjustable block did you try? I'm surprised that it didn't alleviate at least most of the issue. My understanding is that you can dial them down to "zero" gas flow, effectively making your rifle a single-shot. Was there just not enough fine-tuning available?

Sorry for questioning everything, this is just a new issue for me and I'm way behind the learning curve...

Thanks again!

Here's another piece to the puzzle - there seems to be a decent gap at the back of the gas block:





Is this normal? Would anyone else out there with one of these mind checking their spacing? I'm wondering if this in conjunction with the obvious residue at the front of the block indicates a problem:







If anything, I would think that a poorly placed/leaking gas block would lead to short-stroking but the BCG always seems to lock back on an empty magazine so I'm stumped.

Just another observation in my new quest for a functioning rifle.


Thanks again folks - I appreciate any and all help with this!

Amen to this! got to the adjustable gas block a bit quicker (less dinero), but a headache none the less.

I used a JP adjustable gas block. I was able to dial it down to where the gas in my face was minimal compared to previously...but never down enough to shut gas off completely.

So far I have not had any issues in around 4-500 rounds of Federal Gold medal match ammo. Overall I have well over 1000 rounds through mine and possibly nearing 2000.

Accuracy has been exceptional, but once my silencer was approved, problems started almost instantly.

Where are the empties landing with your Mark 3's? Mine land about 3 or 4 feet out at 1:30. It is the only AR I own that doesn't land the enpties at 4 or 4:30 and 10 feet or so away. (I used 2 quarters on top of the delrin spacer the factory put in to stop the stovepipes.) I tried removing the ejector donut and it made the rifle land the epties all over the place, some of them just barely falling out of the receiver.

Mine were landing at the front bipod leg....now my cases are landing around 2:00-2:30 with the silencer and 2:30-3:00 without. Gas in the face suppressed is minimal. My gas setting via JP block is at a minimum and where the gun runs the best.

Planning to run my rifle in the heavy tac optics class in next months Midwest3Gun.....have been playing with it lately to get it ready and function has been 100%.

I could have sworn that I posted this earlier but don't see it. Have a couple of updates:



1ST UPDATE


Well, I sent my non-ejecting MK3 back for repair and they replaced the barrel with an 18" at my request. I have to give credit where credit is due - Chris at CMMG was extremely helpful and the return process was about as painless as it could be. The service turnaround time was amazing (I had it back in my hands 7 days after they received it) and the non-ejecting issue appears to be resolved (mostly). There is no residue around the new gas block after shooting today and the distance from the barrel step is greatly reduced from what it was so it looks like Chris was spot-on about getting that replaced.

I ran 100 rounds through the rifle last week with only a couple FTE's, however the ejection pattern is still pretty strange. Around 1/4 of all the rounds landed on the table less than 6" from the ejection port (almost like they were just falling out). The rest landed somewhere about 5-6 feet out at around 1:00-2:30. It was all new production PRVI so I'm not sure what the issue is. There are also impact marks on the front edge of the ejection port which are definitely from last weeks trip. I'm hoping that this will work itself out somewhat as the rifle wears in - at least it is ejecting now which is a huge improvement.


2ND UPDATE 4/29


While the rifle was ejecting after the trip back to CMMG, I still wasn't happy with the odd ejection and I can't abide a less-than-reliable rifle (if I can't trust it, I don't want it). I decided to go ahead and replace the gas block in hopes that it was an overgassing issue - that's what the signs pointed to and I figured it couldn't hurt. I ordered a PRI Adjustable Block from Midway and swapped it out before this weekends trip to the range. The swap was simple and...

SUCCESS!!!

Started with the port completely closed and gradually opened it up until it was just locking back on an empty mag (1 test round per mag). After some fine tuning with several different ammuntion types I called it a day. Perfectly reliable ejection with everything I threw at it. I went from cases landing at 1:00 or dribbling out to 6'-8' ejection at 3:00-4:00. Instead of random ejection, I now have nice, neat little piles of cases exactly where I want them. No FTE's and no new marks on the front of the ejection port either.

Call me a believer - if you've got ejection issues with your .308 AR and everything else seem to be working properly, start with an adjustable gas block. You may just save yourself some headaches and will likely end up with a better, more reliable rifle.

Thanks for all the suggestions folks - much appreciated!

-DW

What is interesting is that on rifles with similar issues a couple of guys have found that the adjustable gas block was the cure and at least one says the new block had little effect. The guys at CMMG are very polite and sincere but they don't seem to know exactly what the problem is. There must be some small variable that is being overlooked in the rush to get rifles out the door to meet the overwhelming demand. Obviously the solution isn't for the customer to have to put a $100 part on a new rifle to make it function properly. (Or stick a couple of quarters in the buffer.) What does CMMG need to do to make these rifles 100% reliable right out of the box as just about every other rifle on the market is?

I am assuming without removing both and comparing that the ejector spring on the 308 bolt is a stronger spring than on the 223 bolts since the brass weighs almost exactly twice as much. Does anyone know if there is a source for stronger ejector springs?

tight chamber?
dirty chamber with left over debree from milling of the barrel has possible embedded itself into?
head space?
mic a spent case and compair to factory?
had problems with my 308, i bore scoped the chamber and found crap in it that had to be lapped out. i lapped, cleaned,polished the chamber, no ftf/fte sinse.
I see you have a slight gas leak at the block. Mine show,s the same carbon deposits but are of no consiquence.

if you can,,,,bore scope the chamber and have a look. i had to pull the barrel off of my recicver and have a look for myself via a nice HD Snap On bore scope. Its the only way I was able to see the problem


good luck

I dont know whats wrong, but i would like to chime in and say i have the same exact rifle and it has worked flawlessly. The 16" stainless with MOE equipment. And i shoot everything my gun is an ammo whore. So just know they arnt all bad! Hope it gets fixed. The gas block would be my bet though, CMMG doesnt fix it which I would try that adjustable gas block.

The OP described the resolution above.

From what I read and from what the guys at the factory tell me most of the CMMG 308's work just fine so obviously there is an undetected problem with my rifle. CMMG doesn't offer an adjustable gas block as an option or I would ask for one. They are now sending me a new BCG to see if there is a problem there. I and the techs are at a loss to figure out what the variable is that makes an occasional rifle be unreliable.

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