CMMG LE Upper. What am I missing?

[jakk280rem]

About two months ago i bought a CMMG LE upper and completed lower. A few days after buying the rifle, I took it out to do a break in/sight in. Ammo was Federal XM193. I broke down the bolt and lubed everything using Break Free CLP. On round 120, the last round in the fourth mag, the bolt failed to go into battery. After disassembling the bolt I discovered the rear gas ring had jumped its groove and was preventing the bolt from turning in the carrier.
I replaced them with three new ones a few days later.

This afternoon I had a chance to fire my rifle again. Thinking the previous incident was due to lack of lube, I decided to relube after every magazine. Rate of fire was one every 15 to 20 seconds at first while i checked the spotter and made adjustments to my sights. Toward the end, rate of fire increased to one shot every 2-3 seconds. Again ammo was Federal XM193 and again after 110rds the bolt failed to go into battery. Again the rear gas ring. It looks like it has been swaged out into a void in the carrier. Inspection revealed no discernable defects.

This is my first AR. I want to like them. Am I doing something wrong? Is this a warrant issue? Ammo issue? Opperator issue?

 

[jungle]

Looks like a bad bolt carrier and or bolt, try for warranty replacement.

 

[Al Thompson]

Ditto. Contact CMMG. FWIW, this is a new one on me.

 

[wally]

I don't think you are supposed to put lube on the rear facing part of the bolt behind the gas rings. The oils may be detonating secondary to being hit with the hot gases that are cycling the action.

 

[Sky]

Have been great customer support I would call them asap. Rephrase that. I have only purchased from them and have not had warranty issues but I will bet you a cup of coffee they will take care of you.

 

[waterhouse]

Interesting. Please let us know if they figure out the cause.

 

[taliv]

i've actually never heard of an issue like that before. The rings form a seal inside the carrier so i have no idea how they could jump out.

i suppose it COULD be a bad bolt/carrier or bad ring, but the odd marks on the tail of the bolt look very suspicious to me.

how are you cleaning the bolt? are you doing anything to it?

 

[jakk280rem]

how are you cleaning the bolt? are you doing anything to it?

i cleaned the entire gun stem to stern when i got it home. i did not clean it after the first or second range session. i didn't think it needed it. the gun is only two months old and has less than 300 rounds through it.

the only thing that has been done to the bolt/carrier is to disassemble, lube, fire. it has passed the headstand test. the carrier is not so oversized the bolt falls into it.

on a side note: except for the swollen gas ring causing a failure to go into battery, it has fed, fired, extracted, and ejected every round flawlessly.

 

[HorseSoldier]

Interesting. Definitely a warranty issue from the sound of it. I'd be interested in hearing what it gets officially diagnosed as, as that is also a new failure to me.

 

[possum]

if you are in the market for ar internals in the future, look no further that BCM, i hope your issue gest resolved asap, the issue that you are having is not normal fo sure. i would get a new bolt and carrier, you need a spare anyway.

 

[jsimmons]

When I buy a new rifle, I break it down completely, clean everything, reassemble, and use gun oil to lube the fire control group. Beyond that, the only thing I lube is the BCG rails, and I use synthetic axle grease for that. I don't oil any of the BCG internals.

I have well over 500 rounds down the pipe on this rifle, and use it for tactical carbine matches. It's been absolutely flawless.

 

[possum]

Beyond that, the only thing I lube is the BCG rails,

for a match that may be fine but if you start running the gun hard that will not be enough.

grease

great choice for lube, oil migrates, and burns off, personally i use high temp lithium grease. good stuff.

 

[Al Thompson]

Any news?

 

[jakk280rem]

I called CMMG last week. They sent me a prepaid UPS label and asked me to send them just the upper. I hope they find something obviously wrong with it to explane whats goin' on, instead of some mysterious head scratcher. It'll be interesting to learn what the problem was.

I snapped a quick pic of both gas rings before i sent them with the upper and a short note about what was happening.

The bottom ring is from the first incident. It completly jumped its groove whith its tail hanging out over the small end of the bolt.

The top one is from the second incident. It is pictured above. it seems to have swollen.

 

[Al Thompson]

Strongly suspect you'll get a new upper and no explanation.

That being said, I have several CMMG uppers and have been completely satisfied. Good that you didn't have to pay to get your upper repaired/replaced, sort of sucks as I'd love to know what that issue was all about.

 

[TonyAngel]

CMMG is a good company with a good reputation, they should take care of it. For future info, the AR is almost idiot proof as long as you maintain it and lube it. Maintaining it means keeping up with parts that wear. On an AR the biggest wear part is the rings, although the problem you are having is not normal. A properly lubed AR should run at least 500 rounds before you need to relube, depending on the lube that you use. There is NO part of an AR that will cease to run because you got lube on it. There's no magic to lubing an AR. The only way to do it wrong in the sense that you'll stop the rifle from running is to not lube at all.

You do NEED to lube the bolt carrier group. You should lube every square inch of the bolt carrier group. All surfaces where you have metal rubbing on metal should be lubed. Someone made mention of running the BCG dry save for grease on the "rails". Rails, what rails? There are no rails. The bottom of the carrier is cut to work with the magazine. The center really does need lube because that is what the hammer rides on. What keeps the carrier in position is the gas key riding in the charging handle channel. Having 500 rounds down the pipe means nothing. That's just a few hours of shooting. When you have 5000 rounds down the pipe, I'd like to know what the track record was.

What do you get when you mix grease with sand/dirt? You get lapping compound, which is used to remove metal. An oil type of lube lubricates and helps to displace crud, not hold it in place. Grease is fine for the trigger group, but not for lubricating the working parts of the rifle. If you just really want to run the rifle as dry as possible, at least lube the rings, the lugs and any surfaces that get shiny. Those are the wear points.

Do not buy into the crap about an AR needing to be clean to run It only NEEDS to be lubed to run. I've had my ARs well into the thousands of rounds without being cleaned and they run flawlessly.

 

[DBR]

Gas ring damage like you've got looks like a defect in the hard chrome plating in the bore of the bolt carrier or some other defect in the bore.

Breakfree brand CLP is an adequate lube for an AR. Walmart sells it where I live. Personally, I like Weapon Shield CLP it is a much better lube in my experience.

The normal way to lube the bolt carrier is:
-one drop of lube down the gas key
-one drop in each of the vent holes on the side of the carrier
-a bit of lube on each of the four carrier rails
-a drop on each side of the cam pin
-a bit of lube on each side of the cam pin head
-lube the bolt lugs

Wetter is better within reason!!!

The carbon on the bolt tail is mostly from propellant gas that enters the gas key and is unavoidable.

The bolt carrier is not guided by the charging handle. It is guided by the cam pin head riding in a groove in the receiver. I make it a point to lube this inside groove in the upper receiver as well.

 

[TonyAngel]

Sorry, I don't want to get off on a tangent here, but it is NOT the cam pin that guides the carrier. I did do a bad job of representing where the gas key rides, but the cam pin relies on the bolt not rotating so that the cam pin can do its job of rotating the bolt. The carrier is guided and kept from rotating by the gas key.