DPMS CMMG PROBLEMS

[98Z5V]

I don't think it's necessarily a mag problem, but a machining problem with the uppers.  Or lowers.  Height-over-bore kinda thing.  Uppers are being machined with the bolt carrier circumference too high, over the top of the mag, or the lower-to-upper stack height is being machined too high.

There's no standard to follow for machining these rifles.  Nobody follows a "set" set of plans during machining.  When we combine different uppers and lowers, the true flaws come out. 

With guns having bolt over run problems in .308, that's the first thing I think of. Different uppers and lowers combined, and the machining differences between the two companies isn't even compatible - because they're all doing what they want to do.

What upper does CMMG recommend using?  Does that upper run with Pmags?  Now, I'm not defending Pmags here, I'm just trying to think of ideas that might lead to the real issue.  There are so many guns that Pmags run perfectly fine in - mostly uppers and lowers from teh same company, though. 

Just a thought...  <dontknow>

[survivalshop]

I think your spot on about the dissimilarities in manufacturing.

I never thought it was a Magpul mag issue & was thinking it was in the lower ,now it may be the upper compatibility issue ,as you say.

I will switch uppers next time I shoot & use my DPMS upper on the CMMG & see how that goes .

I have an SI Defence upper with it now . I'm still going to measure all spec's on the lower, & upper also.

Ya know if the mags work with a DPMS upper , I will have an SI Defence upper to start a new build .

How do they say it , a silver lining in every thing .

[the bigness]

My problem w/bolt ride over is w/ p mag/DPMS upper/CMMG lower and Calif. legal bullet button which may be the entire problem.

[survivalshop]

My problem w/bolt ride over is w/ p mag/DPMS upper/CMMG lower and Calif. legal bullet button which may be the entire problem.

I see a lot of rifles with that bullet button ,that have feeding problems . Wish I knew some thing about that contraption. I guess I will have to look into it .

So , my problem with my CMMG lower may be a different beast, good I'm up for the challenge .

Ya never know , me finding some thing about my CMMG ,could help you with yours.

[survivalshop]

I looked into that "bullet Button " & I can see no problem with its function , since you are using the normal mag catch .

The only thing I can see that could be a problem is spring pressure on the catch , but that could also happen with the stock set up .

I need to look into the receivers & there specs'

[Brassjunkie]

I installed a Fulton Armory BCG and it works better that the DPMS because the bolt face lugs arn't beveled so much and it picks up the round more reliably

However I noticed the mag slop does have an affect on both DPMS and Magpul mags if your holding the rifle by the magwell while firing. Shooting off a bipod it fed fine. I need to try another brand of mags to see if I can eliminate mag well slop. Also tried this on a SunDevil lower and had the same issues as the CMMG.....The problem is the bolt face and mag slop.

[98Z5V]

Can you grab a couple comparison pics of the bolt faces, and post them in here? 

[survivalshop]

I would also like to see a comparison of bolt faces , because I thought Fulton bolts & carriers were also DPMS .

Its not past the folks at Fulton to modify a component to make it work better . They are very good in what they do with rifles .

[Brassjunkie]

I'll post some pics when I can get some to turn out right....still working on it bear with me.

[Brassjunkie]

Here are some pics of the dpms and fulton bolt face. The dpms is the bolt only, the fulton is with the chrome carrier. The fulton bcg is better at picking the rounds up, but will still slide over if the mags are pulled back. If you push the mags forward they work fine. With that being said I think mag slop is the most contributing factor in the failure to feed issue. I'm gonna try a fulton mag next and see how that works.

[survivalshop]

Now I have to look at my two bolts . One is from Fulton & the other is a DPMS .

[survivalshop]

I took some measurements of what I think they would need to be taken of the CMMG & DPMS lowers .

I basically wanted to know cartridge height as in a mag into the lower .

I measured from the top of the receiver deck to approximately to the top of the base of a 308 rd. ( used my dummy's I had made up ).

The measurements are rough & we can question my tools or the operator (me ) , but through my measurements ,I noticed a pattern.

I also took a rough measurement of the distance from the top of the mag feed lip to the top of the stop on the mag where the catch hooks into it .

I figured that a full loaded mag of twenty rds. ,the mag would seat there because of the weight of the mag..

MAG    LOWER RECIVER MAKE        MEASUREMENT

Magpul----------CMMG  ------------avg. .0400 "

CProd.  ---------CMMG  ------------avg. 0.406

DPMS  ----------CMMG  ------------avg. 0.408"

Magpul  --------DPMS  -------------avg.0.409 "

CProd. ---------DPMS --------------avg. 0.419 "

DPMS  ---------DPMS  ----------avg.  0.443 " (?)

Distance from the mag.s' stop or catch grove to feed lips .

DPMS ---1.190 "

CProd. --1.173 "

Magpul --1.143"

IMO , the Magpul ,Pmag and the different spec.'s of the CMMG lower compared to the DPMS ,that I will not be able to use the Pmags in the CMMG lower . They don't work now ,so that's not a big deal .

The bolt over problems ,I have had since I started to use them in the CMMG Lower, it seems that only about 0.010" can make a difference in if they work or not .

The measurement of the distance from the mag catch grove  to the top of the feed lip also got me scratching my head . I know there is a material difference between the Pmag & a steel one , but that's a big difference.

I also know that my CMMG lower receiver can be one in a 1000 to be out of spec's ( not to mention different uppers ),but since there are none ,so to speak ,for 308's, its just the way it is .

Makes me want to investigate any manufactures fitment.

[the bigness]

So what should I do, sell my lower online and get one that works with p-mags? Any suggestions for OLL in California?

I got good @ black ops,but I'm not a kid and I have to much $$$ in the real thing.

[survivalshop]

Have you tried a DPMS or CProducts mag yet ?

They work flawlessly in my CMMG lower .That's all I had & used until the Magpuls came out & I had no problems with the Magpuls in my DPMS lower ,but as soon as I tried to use them in the CMMG lower ,I had bolt over & I thought it was the ammo I was using ,milspec 7.62.

If you need one of each to try ,I can let ya use them to try out . I still have new in the wrapper of both & no not for sale , but you can try them if you can't get your hands on some .

[the bigness]

When I put it together I bought 10 dpms 10 round mags and non of them worked. I had read that you can modify them but I wanted the thing to work. I still have them. Next I bought 10 p-mags 10/20 and they don't work either. I have yet to try cproducts.

All I shoot is milspec NATO rounds.

[98Z5V]

My problem w/bolt ride over is w/ p mag/DPMS upper/CMMG lower and Calif. legal bullet button which may be the entire problem.

At this point, this is the entire problem. 

I'm not telling you to break the law - I'm telling you to get your ass out of state, remove that thing, and see if your rifle functions at all with that thing out.

The only out-of-norm WAY screwy piece of the puzzle is your bullet button.  Come visit me in AZ, let's shoot.  Get that thing (legally) out of there and try to eliminate one more potential problem.  When you say you can push up on the mags and get no problems - that screams mag release. 

[survivalshop]

At this point, this is the entire problem. 

I'm not telling you to break the law - I'm telling you to get your ass out of state, remove that thing, and see if your rifle functions at all with that thing out.

The only out-of-norm WAY screwy piece of the puzzle is your bullet button.  Come visit me in AZ, let's shoot.  Get that thing (legally) out of there and try to eliminate one more potential problem.  When you say you can push up on the mags and get no problems - that screams mag release.

That's exactly what I was saying in the post about cartridge height .  I have not tried to push up on the Pmag to work & it might ,but that's not the way its supposed to work .

MY CMMG lower 'ed rifle is having the same problems & I don't have a bullet button.

If you look at the measurements  above , it seems not to have to much off for a mag, not to work (in the way CMMG  receiver is made, or at least mine & maybe his )

If you look at the distance of where the Pmag seats latched in the mag well , its lowering the cartridge & may be making it too low for the bolt to grab it .

Nothing against Magpul ,because the same mags work fine in my other 308 ,its something in the machine work of the CMMG lower & magpul combo , that's not permitting them to function properly.

After looking that bullet button contraption over , there is no difference on it & the std set up . The mag is still attached with same components. None of that is changed , the only thing changed is the way the mag release is activated .

We all know some  DPMS have feed lip problems . I have had to alter some of mine ,to work .

The  bigness ,Pm me with your address & I will send you a CProducts mag for you to try or we can go over how to adj. the feed lips of your DPMS mags to get them to work .

[98Z5V]

Bigness, do you have a standard mag release?  Maybe take a few measurements from the actual catch-part of it that holds the mag in place - the square pad that sticks into the magwell.  I can get a measurement from the DPMS .308 catch, and several different brands from my 15s.  If the square part of that catch is "thinner" top-to-bottom, then it won't hold the mag high enough in the magwell.  I don't think that would have anything to do with the bullet button, just the mag catch. 

It's worth a shot.  <thumbsup>

[survivalshop]

Ya know ,that's a good idea & I just thought of some thing else , what if there is a larger window where the mag catch sits in the receiver & is too sloppy ,letting the mag catch drop too much with the weight of a full loaded mag ..

When I first started to use Magpuls & had problems , I could go up to five rd.s before it started to screw up. After five rounds ,I would have FTF & bolt over .

If the weight of the mag . is pushing down on the mag catch & the window in the receiver is got some room to let it drop down enough to cause problems.

I am going to measure mine also & see if it drops down at all & how much.

[survivalshop]

I took some measurements of the mag catches from my DPMS & CMMG rifles & they are exactly the same .

Both have a little slop when assembled in the receiver , as far as up & down play , I would say about the same & can measure it if needed , but its not enough to be a concern .

Thickness ,meaning top to bottom as its in the receiver , 0.2445 "

Width , from front to back , as its in the receiver , 0.2975 "

Now here's the kicker , I measured from the top of the opening of the mag catch in the receiver, to the top of the deck of the lowers,

DPMS --0.659"

CMMG --0.669 "

That puts the mag in the receiver 0.010 lower than the DPMS .

Couple that with the distance from the top of the indentation where the mag catch is held in place in the mag. , to the top of the feed lip.

Magpul  -----1.143 "

DPMS -------1.190 "

CProducts ---1.173 "

That puts the  Magpul mag even lower in " my" CMMG lower.

WE have talked about tolerance stacking , well there it is .

I see now way , sort of raising the mag location on my CMMG to permit it to reliably feed with Magpul mags . As said I have no problems with the DPMS or CProducts mags in this lower .

Its not the Mag. or the catch , its the receiver , as far as my CMMG lower is concerned & I'm not down on Magpul or CMMG , after all ,CMMG was out before Magpul mags were out .

I may have a badly done lower ,but she does run with other mags , so I can't even say that .

[survivalshop]

Took both the 20"DPMS & 16" CMMG to the range yesterday  & switched uppers around to test The magpul mags.

Put the 20" DPMS upper on the CMMG lower ,installed a 20 rd. Magpul mag & it fired about two rounds & had bolt over FTF.

Reinserted the mag & it fired one rd. & same thing .

Put a 20rd. Cproducts & dumped the entire mag, no problems .

Inserted a 19 rd. DPMS mag ,dumped all 19 with out a hitch.

Put the 16" upper (SI Defence upper receiver ) back on the CMMG lower .

20rd. Magpul , FTF first rd. ,got  it to feed a round & had bolt over FTF second round .

Installed 20rd. CProducts mag , dumped all twenty with out a problem .

Installed a 19 rd. DPMS mag & dumped all 19 with out a problem .

All ammo used was milspec XM80C.

I just had to test this ,even though I knew what the out come would be before hand , but hay, ya don't really know till you try it .

Note :

And by the way , the Magpul worked fine in the DPMS 20" all back together.

[Jgun]

Is the magpul fairly new, did you try any other magpul mags? I was told by someone that swears by metal mags, that the magpuls can wear out in the mag release notch and sit lower, Although I've yet to have a problem with either the .223 or the 308 magpuls in my guns, same thing for the SR-25 mags I have, they've all worked 100%. The only mag I've had problems with in my AR's has been an extended DPMS that for some reason locks back with one rd left in the mag every time.

[the bigness]

Well it looks like Magpul is to blame. I have every possible magpul product on this rifle but if it takes c-products to make it go than that s what Ill do. thanx for your help and keep me posted if anything  comes up.

[usadaytrader]

I have the same bolt over problems with my CMMg and DPMS upper.  Has anybody resolved this?

[survivalshop]

In my case with the CMMG 308 lower , its the Magpuls do not work with there old lower receiver design .

They have a new type on there new rifles they are selling & the Pmag works well .

Any steel mag I put in her works fine , just not Pmags.

For all I know they ( CMMG )might of had a bad run or lowers when they first started to make them .

Have you tried different mag's , including other manufactureres ?