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(First post) Ejection problems on MK-3


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Hello all,

I just purchased an 18" barrel MK-3 from my local store. It's my first AR-style rifle (although not my first gas gun .308). I'm a bit of noob to this platform, but am having some problems. See if you can figure out what's going on. Any help is appreciated.

Here's the gun:


Beautiful, ain't it?

First session I shot around 40 rounds through it, and it seemed to leave the last round in the ejection port when the mags are finished. It did this about 3 times, after emptying the mag about 6 times.

[img width=810 height=607]http://3.bp.blogspot.com/-lJJO0u4h6Ik/T5dQkE8Z4sI/AAAAAAAAAf0/WCcI8YsJjTo/s1600/last+shot+case.jpg

No big deal, technically, since I am about the swap the mag at that point anyway and the round just falls out.

The ejection seems to be a little weak, since the cases more-or-less stay on the shooting bench with me after they are shot. I can even put my duffel bag next to the gun and it flips them in there like it's flipping a cigarette. Standing, the cases probably go 3 feet. I thought, "maybe this is how all ARs eject."

But then this happened:

[img width=810 height=607]http://1.bp.blogspot.com/-jydzs088zwY/T5dQgMTUlaI/AAAAAAAAAes/2-M6klbc_Ck/s1600/Jam+1.jpg

It's pretty darn stuck in there. It took some banging to get the case out of there.

Here's a shot of the jam with the bolt pulled back:

[img width=810 height=607]http://4.bp.blogspot.com/-zEDdqXyIOJI/T5dQjmiFYCI/AAAAAAAAAfs/UzehXVVLnjo/s1600/jam+2.jpg

The case is really stuck.

I took it to my local store last weekend to install a new scope mount, and I mentioned the jam to the clerk. He just threw a bunch of oil on there, and said "Keep running it. Get a few hundred rounds through, then worry about it."

Good advice, I think.

The clerk also checked the spring pressure on the ejector to make sure it seemed strong enough. He did not strip the bolt, as there is a threaded (?) pin which keeps the bolt from being disassembled as easily as a normal AR-15.

So today I took it out put about 20 more rounds through it. Once again, the last round was staying in the ejection port. No biggie.

Then, THIS:

[img width=810 height=607]http://i.imgur.com/A6aZv.jpg

This is one of two jams I got today in 20 rounds. ARRRRGGHHH!

I know that there are brass kisses on the brass deflector, so the rounds that are ejecting normally are doing so hard enough to leave marks. That's good. But the cases seem to bounce around a lot in the port and have a tendency to stay there while the bolt comes forward. They even bend the live round below it slightly because they hit so hard.

You'll notice that one jam is face in, and one is face out. The bouncing is obviously pretty random.

Any ideas? I could replace the extractor, but I don't know if it's too weak or too strong. Any way that you know of to test if the extractor is working???

For reference, I was shooting German MEN ammo today, and at the other range session. That's what it jammed on. I shot a little Federal 150gr soft point and some milsurp PMC.


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Ok, so it only happens on the last round in the magazine?  What are you using for mags?  Does it happen on all of them?

I'm no pro, but some food for thought :

Milsurp and 7.62 ammo, even lighter rounds may not have enough to cycle the gun properly, cheap and easy experiment is try some 168 gr American made .308 rounds.

It's new, run her sloppy wet for break in, probably not the root cause though, but will help

Maybe you need a heavy buffer to slow down the cylic rate, although since its only on the last round maybe not. There will be more help here soon. Try ammo first.

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Welcome from Indiana brother Caveman,Got a RRA Lar-8 that for the first 400 rounds never had a hiccup ran flawless,german mil-surp,and factory,any weight then got 147 gr. Belguim mil-surp and had FTE 5  :( out of 40.Got 210 to go will just brun them up.poop ammo,garbage in garbage out.Unless I get Dag or Men done with mil-surp.

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Hi Caveman !    I always start  my detective work by putting one round in a mag and see if it locks back the bcg on that one round...start by doing this...also on a new tight chamber I always use .308 ammo and not milsurp .762...although I do like the Dag and Men...but you gotta get the bugs worked out before you go to that stuff. Sometimes folks think that they are undergassed when in fact you can be overgassed and get these problemos...so first off shoot one round mags and see if it locks back, run er wet, shoot some 308 federal match ammo or sumthin like it, check for any flecks of metal in the bcg.maybe try another mag...and get back with us  went back and read your post again....rounds aint flying out....sounds undergassed...weak ammo,new tight rifle,mag,maybe mis aligned gas tube/block,but thats getting ahead of things

:)  Wash

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Some clues and clarification:

--It locks back the bolt perfectly fine on the last round, so that tells me it's not short-stroking. Not under-gassed.

--It is not the last round that it jams on. The case is just LIKELY to stay in the chamber on the last round of a magazine-- about 30% of the time. It doesn't make a malfunction; the case just sits in there until I drop the magazine and it poops it out through the magwell.

--I'm using a Pmag. I only have one magazine because I just bought the rifle.

This guy had the same problem:  http://www.ar15.com/mobile/topic.html?b=3&f=121&t=569850

And it was solved with a new gas block: http://www.brownells.com/.aspx/sid=102711/pid=7592/Product/AR-15-M16-ADJUSTABLE-GAS-BLOCK?utm_medium=shoppingengine&utm_source=googlebase&mc_id=10000&gdftrk=gdfV21820_a_7c187_a_7c3466_a_7c452000148_d_452015110_d_10868

Per the splash page of this website, most AR-10s are over-gassed.

Here is my best guess at the moment (and it is DEFINITELY A GUESS): The rifle is over-gassed, causing the bolt and carrier to travel too far back behind the back of the ejection port. The case begins to exit the ejection port but is bounced back in by the rear of the port. Depending on the orientation of the bouncing case when the bolt comes forward, the case gets slammed up on top of a live round. Note that in one malfunction instance the case was facing one direction, and in another malfunction it facing the other direction. Entropy!!! :-)

This would also explain why the cases come limply out of the ejection port and land so closely. Most of their inertia is taken up by hitting the rear of the port. (Maybe??? This is only a theory.)

My current solution is to get a new gas block and dial the gas down. What I wouldn't give for a slow-motion video camera to watch it shoot.

(By the way, why don't all gas guns come with adjustable gas systems? My FAL had one that worked great, and that's a 50 year old design. Seems logical.)

Thanks for the warm welcome, everyone. I'm hoping to make this work with German DAG and MEN since I'm swimming in hundreds of rounds of this stuff.

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Hi Cave  Yeppers you're on it....she locks back...good !

overgas prolly so.Did you get to shoot some .308 in it?

Ive shot Dag in two new builds of mine with not a hitch right off the get go...just to see what they would eat.I guess im lucky...ive built two mega ma10's for me and one for my Bud...and have never had a cycle problem on any of em . Im sure other members here with more knowledge than me will chime in :)  Wash

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PRI low pro adjustable that you've pictured, has a set screw that you screw in to restrict gas from the barrel port. I have not used the JP, but do like the PRI because of it's small size (fits under most FF hand guards) and the fact that it's steel and not aluminum.

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Seeing that you have an 18" barrel with a carbine gas system almost says you are over gassed by itself. It amazes me that manufacturers are still using carbine systems at all.

The adj gasblock is one of multiple options you can choose. What you main objective is, is to slow your cyclic rate. This can be done by limiting the amount of gas through the action, adding weight to the carrier, or improving the buffer system.

Your first answer (from the gun shop) was good sound advice. 308 AR's like to be sloppy wet when they break in. You next step should be measured carefully.

The adj GB's can work, but if this rifle is one that your life is going to depend on, I never suggest this option. The trick of tuning the GB is that you are going to be shooting one type of ammo and you expect little to no variation in bullet performance. Seeing that you like Mil Surp ammo, I think the adj GB should be your last option.

A Tubb Carrier Weight System or a HEAVY BUFFER with a power spring would be an easier, and more likely beneficial, remedy.

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Ok, I think I see how that heavy buffer and spring works. Should I get the 5.6 oz one, or the 6.5 oz one?

Doesn't it seem to be addressing the problem of over-gassing with simply a "bigger hammer"? Wouldn't it make more sense to get the correct amount of gas to the gun, rather than letting it bleed everywhere randomly, and stomping the motion down with a heavier spring and buffer?

(Sorry for the noobish questions. I've never had an AR before.)

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Oh, and another theory question:

Am I right in thinking the bolt is going TOO FAR back, causing the ejection problems? Why would a bolt be designed to go too far back to allow ejection? That seems like a crummy design, if that's the case.

Shouldn't too much gas still allow the darn rifle to eject, albeit with more wear and tear on the parts?

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The problem is that both the .308 and the 7.62x51 create a LOT of gas. The AR's  are still using a gas system that was designed for the 5.56x45 and .223. Even the AR-15 is getting away from carbine gas, unless barrel length forces it.

In reality, you should have a rifle length gas system on your 18" barrel, or at the very least a mid-length. A .308 requires 15.5" of barrel to make a complete burn. If you have a gas port at 7"-9", you are pushing a lot of burning powder through the gas tube. It doesn't matter how much you restrict the gas flow with an adj GB. The burning powder still creates expanding gasses inside the gas tube.

The more expensive, yet appropriate, correction to your rifle would be to contact Fulton Armory and look into one of their 18.5" rifle length barrels... Or even better yet THIS ONE.

The FA barrel would allow for more of the powder to burn in the barrel, rather the gas tube. This would be a better regulation gas than the adjustable gasblock. Unfortunately this a $300 fix vs a $50-$100 remedy.

As for the BCG traveling "TOO FAR", that's only possible if you had the wrong buffer and buffer spring installed. Even then, you should notice that a full compression of the buffer spring is occurring when the buffer slams back hitting solid resistance... As long as you have .308 specific parts installed, I don't think this is happening. I think the bolt is traveling so fast, that it is already returning to battery before the casing has left the receiver.

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To add to what has been previously posted. It shoudn't be how far back the bolt carrier is traveling, it's how fast. If the case doesn't have enough time to eject before the bolt passes the back of the port, it could be hanging up. Another thing that sometimes happens when the bolt travels too fast in the other direction, is that it can override the next rd when traveling forward, and you get a failure to feed. I have not done a lot of testing with different spring/buffer combinations (I'm sure others here have) I have installed Slashes Xheavy buffers in my guns with excellent results. One of the things that sometimes happens  with an adjustable gas block is that if you set it to JUST cycle a certain rd, you end up with reliability issues when switching to other less powerful ammo. If you plan to use the rifle for self defense or hunting dangerous game, reliability becomes paramount. You might  also want to  check your extractor/ejector for proper function..

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Your buffer tube needs be wet & slippery inside,  also.  Very smooth consistent operation of the buffer.

Hoot posted a method for adding weight to your BCG in this forum also (somewhere!).  Alternatively,  heavy factory carrier or heavy buffer.

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... One of the things that sometimes happens  with an adjustable gas block is that if you set it to JUST cycle a certain rd, you end up with reliability issues when switching to other less powerful ammo. If you plan to use the rifle for self defense or hunting dangerous game, reliability becomes paramount...

Bingo BANGo! <thumbsup> The method to adjusting a gasblock is to open it up completelty, then adjust the restriction till your rifle stops cycling. Once you find that breaking point, adjust it back 1/4 turn for reliability. That's not my idea of reliability, especially since anything "adjustable" has the ability to come maladjusted.

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OK, excellent input all around. I'm starting to get a picture of what is happening.

---TOO FAST is the issue, not TOO FAR.

---Adjustable gas block can be unreliable compared to other solutions.

---There is no "proper gas" in 308 ARs with give reliability across ammo  like there is in M1As, FALs, AKs, etc. It's simply building an autoloading system that is properly timed with the majority of what you might fire through it.

---There are no .308 gas blocks.

Actions: I will be getting a new buffer spring and buffer. Anybody know what weight (5.6 ounce vs. 6.5 ounce) I should get?

BTW-- Crap, those are expensive.

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Hey Cave  What Robo said!  Daaamn I missed the part bout carbine gas length tube...its overgassed for sure...I wouldnt get an adjustable gas blcok...id go like Robo sez...put the attention on the other end...the buffer/buffer spring to slow things down :)  Wash

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Your right they are expensive, and it may be possible to solve your problem cheaper through other methods/trial and error. I'll tell you this, when I take the 50 mile ride to my range, I'm really pissed off if my gun doesn't run. Keep in mind, if this one thing will make your gun run for you, it would be money well spent, if it doesn't, I know your going to be really pissed, but you should be easily able to sell the buffer on this forum, probably to me.  If I were you, I'd give Clint a call at Heavybuffers.com and ask his opinion about the problem, and what buffer he recommends, good luck.

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So on the way to the gun store this week, I called CMMG. A technician named Josh said he had read this thread. (I sent him a link by their technical support form on the website.) He said that CMMG is working on a solution to it, but indicated that they are aware of the problem with their rifles.

He mentioned putting a few quarters in the buffer tube. He also said a heavy buffer might fix it as well. I asked him if they had any heavy buffers laying around that he could send me, and he said he would look. That would be nice-- a company that sends parts needed to make their product work.

Josh did not have any specific solutions, although he was very polite.

So I went to the gun store, and told the clerk my problem. He had heavy buffers in stock, but they are too long for a .308 gun. (The bolt and carrier are too long). So he opened up the buffer on my gun, took out both the blocks of tungsten that are inside, and put a thin layer of steel shot between them. It's probably only a fraction of an ounce more weight, but it can't hurt.

On the way back, Clint from Heavy Buffers called. He explained that I need the CAR-10 buffer and his modified Armalite springs for $125 including shipping. I'll probably order that when I find some cash. He said that CMMG probably uses standard DPMS buffers in their guns, which don't work very well.

So I took it to the range. What follows is a pictorial report of shooting German milsurp, Russian steelcase, and Remington Ballistic tip ammo.


What a beautiful day to shoot!!! (Well, it was pretty humid, but I just love being at the range.)


I'm running this thing WET. Like a swamp.


This is the grease I am using.


Second shot of the day. German MEN ammo. I guess the steel shot added to the buffer didn't do poop.


This is the NEXT round which was partway in the chamber during the jam. The bullet is canted in the case. I threw it away.


So I switched to trying some of this Herter's steelcase ammo I have laying around. I got lots of this stuff, and it would great if my rifle ate it.


It basically went where I aimed it, which is a good sign. Although there was a random flyer here. So far so good, and no jams.


...but it's STILL leaving the last round in the port, SOMETIMES.


The Herters ammo didn't stay tight, though. This is a big-ass crummy group.

Suddenly, a big boom and smoke is EVERYWHERE, pouring out of the gun. You should click the link below to see what I am talking about.


Watch the above video to see smoke leaking out of my magazine and mag well.


Definitely a ruptured case head. To be fair, I had this happen one other time with my old rifle, an FN-FAL, with this same ammo. The FAL took it like a champ and kept on chuggin'. This one did the same. It even fed the new round, no problem.


The ruptured one went pretty high. But that's within the margin of error for this stuff, anyway.


On to some better stuff, the Remington .308 Ballistic Tip ammo has been laying around for a while. We'll see what happens.


Didn't group very well, and slid 4 inches to the left. Of course, we couldn't even make this crap make a group with my wife's bolt gun, either. So my standards were low. Don't buy this ammo.


I'm still not getting what I would consider "healthy" ejection from the gun. Here are the second to last (on the mat) and last shots with the Remington ammo. 6" ejection seems to be par for the course.

I'll let you guys know when I order a heavy buffer, and how it affects the gun. Thanks everyone, for your help so far.

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My Mark 3 is just like yours and I had the same problem.  I sent it back to CMMG and they put a delrin spacer behind the buffer spring.  It didn't help at all.  (One of the guys on AR-15.com suggested quarters on top of the spacer to keep the BCG from going as far back.  I passed that on to Brent and Josh.)  That helped but I was still getting the same FTE's on about 1 out of 6 shots.  CMMG sent a new BCG and I put 20 rounds of mixed ammo through it today before the storm hit.  So far it is working.  They told me they had some carriers that were out of spec.  It is a lovely rifle and fairly accurate.  If it will be reliable it will be great.  I'll shoot a bunch more in the next couple of days before declaring the issue fixed.  I think the guys at CMMG while very nice, are a little mystified too.

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So I went to visit my friend today who has a CMMG MK-3, but a different version. His is having no ejection problems. He has only shot about 40 rounds, but they appear to travel about 9 feet standing, and 6 feet prone.

We took our guns apart and started comparing parts. Some interesting differences were noted.


First and foremost, I noticed that I am having impact marks on the front of my buffer tube. The bolt is slamming into it making marks on either side. They are highlighted in the above picture. That can't be good!!

(Sorry I didn't have my good camera, just a cell phone.)


Secondly, his buffer is CONSIDERABLY larger than mine. And it obviously works, since his gun works and mine doesn't.


His buffer spring is also longer, and thicker.

My friend seems to have the version of the gun that CDNN was selling last year in their catalogue, including a 18" heavy barrel, slotted handguards, Magpul PRS stock, and 2-stage trigger. Here is a comparison of the two lowers:


So I thought to myself, "It's time to stop screwing around. Let's try to fix this."

Taking Josh from CMMG's advice, I dropped 3 quarters in the buffer tube and re-assembled the gun. I stepped outside to the backyard with 3 rounds in the magazine...

The first and second round still demonstrated rather anemic ejection. The third (and last) round did not stay in the ejection port, but fell at my feet. The bolt locked back.

...And the bolt hold open lever promptly broke.



Well, now I not only need a solution to the ejection problem, I need new parts to even fire the gun.

The bottom of the lever snapped right off. I found the small pin in the grass, but couldn't find the broken half of the lever. Not that it would matter, it's broken right in half.

[img width=810 height=607]http://i.imgur.com/oig8xl.jpg

So that's where I'm at.

If CMMG had this rifle working LAST YEAR, why did they change it? Pretty strange. They obviously have a longer buffer tube, a tougher spring, and a much longer buffer on the guns they sold to CDNN. Why mess up a working design?

Josh, if you're reading this, you have my mailing address. Please send a new bolt hold open lever and a heavy buffer to me. Alternately, send the buffer system that my buddy has. His seems to work great.

In case you didn't notice, I am seriously bummed out. Someone needs to make this right...

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