DPMS LR308 failure to extract

[chpprguy]

On my first outing with my DPMS it shot about 40 rounds and then ripped the rim off of the fired case...

I took it home, used a cleaning rod to get it out.
I assumed it was just dirty, so I cleaned the chamber/barrel/bolt/receiver very well...

out today,shot about 60 rounds (not all at once)
same thing, ripped the case rim off....

possible causes?

shooting NATO milsurp... (lithuanian 147gr)
that fires fine in my M1A and FAL....

 

[Hummerdawg]

Re: DPMS LR308 failure to extract

sounds like you are over gassed, maybe time for a adjustable gas block

 

[Strangedays]

Re: DPMS LR308 failure to extract

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Hummerdawg</div><div class="ubbcode-body">sounds like you are over gassed, maybe time for a adjustable gas block </div></div>
Over gassed with NATO? I don't know if that is it.

 

[chpprguy]

Re: DPMS LR308 failure to extract

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Hummerdawg</div><div class="ubbcode-body">sounds like you are over gassed, maybe time for a adjustable gas block </div></div>

ok.. thanks...

 

[SScott]

Re: DPMS LR308 failure to extract

There is a great read on AR15 dot com, on the operating cycle of the AR.
It sounds like your bolt is moving rearward before the brass has had time to blow all the powder and start to contract off of the chamber walls. I agree with the idea about getting an adjustable gas block, might be a step in the right direction, if you own a pile of that ammo.
Good luck on the DPMS, I am loving mine and am finding out just how accurate gas guns can be.
SScott

 

[JSTARSZ]

Re: DPMS LR308 failure to extract

it does seem to be out of timing. The bolt is moving before the cartridge is completed in the chamber and that is causing the violent ejection issues. So it needs to take a trip to DPMS and let them fix it.

If the ammo works in other rifles that is not the cause. It is the rifle's gas system.

 

[ajwarrior]

Re: DPMS LR308 failure to extract

Just curious if you were following the recommended break-in procedures. I spoke with JonF at DPMS before I took mine out for the first time and learned it is important to make sure during break-in to have it very wet.

 

[kilo7788]

Re: DPMS LR308 failure to extract

Dont listen to the folks at DPMS. Most of them know nothing as was proved to me while talking to them about a similar problem. Is your barrel a .308 or a 7.62. What model gun are you shooting? I have read alot about the chambers on the DPMS needing to be polished before they extract and eject properly. Also as said about they require more oil than an old truck. The starting to extract while it is still (firing) is somewhat of a theoritical problem that came about when the M4s first started coming into service. But I belive that it is BS becasue the GAU and the GUUs have been shooting fine since the 60s and 70s. If you want to PM me I can help you check a couple things. For additional help go to the DPMS forum and just search for similar problems.

 

[ajwarrior]

Re: DPMS LR308 failure to extract

JonF has given me solid info and advice on my lr308. Like kilo said they do require lots of oil in the beginning and yes some polishing may be necessary. Just make sure your lr is cleaned well and super wet. And more creds to kilo for advising on the DPMS forums, have been on their forums for a months now.

 

[chpprguy]

Re: DPMS LR308 failure to extract

the model is a LR-308 caliber 308 (stamped on the lower)
I see no markings on the barrel, i'll remove the sight and handguard and look for markings.....

this model....

I am not the original owner, so I am unsure of the break in or care of the rifle, prior to my owning it.

I will clean it again thoroughly and look at getting an adjustable gas block... and inspect the chamber as best I can,
I need to get the appropriate tools to remove the barrel...

I may just turn it into a 7mm-08, I'd just like to get it functioning correctly first...

 

[Unknown]

Re: DPMS LR308 failure to extract

I had similar problems with my DPMS LR308. Stuck cases, bullets jammed into the throat...I eventually decided that it had a short throat that caused excessive pressure and the function problems.

I returned the rifle to DPMS for repair. They said they had feeding problems and replaced the magazine. I had never had, nor did I ever report any feeding issues.

Funny thing is that the rifle now works like it should. I suspect that they fixed the short throat, and simply couldn't bring themselves to admit to a quality control issue. Rather than admit that their throating reamer wore out prematurely, they appear to have fixed my rifle and lied about it. I no longer have the rifling engraved on bullets of rounds that have been chambered but not fired.

Whereas I'm happy that the rifle runs normally now, I am very disappointed in being treated like I'm too stupid to recognize that they fixed the precise issue I complained about. It makes me not trust them anymore.

You might want to check for a short throat...my rifle also ripped the rim off cases that were stuck in the chamber before the rifle was returned...it hasn't happened since I got the rifle back...what would you think?

 

[CDAT]

Re: DPMS LR308 failure to extract

chpprgy, FYI and consideration, I had the same issue, extractor ripping case rims off, with my DPMS LR 308. However, it would only do it with reloads, never with new US made or US MilSup ammo. I installed an adj gas block and started using a small base resizing die on the cases I reload and shoot in the LR 308, After that the problem went away with one exception. When using Federal small based sized cases it would still sometimes rip a rim off. I have been told that Federal brass is soft (could be an issue with your brass/ammo also)and I may not be getting a true SB sized case when I process it. The alleged brass softness may have been affecting the rate and amount of case spring back, causing the case remain sealed to the chamber longer after firing. Now I only use small based sized LC and Win Brass when reloading for this rifle. I was also told, when I purchased the rifle, that DPMS 308 chambers are on the small end of the tolerance scale.

CDAT

 

[rcw3]

Re: DPMS LR308 failure to extract

I would try different ammo first to make sure it's not something weird with Lithuanian ammo.

Next I would polish the chamber which can be done without taking the gun apart and can be done with a cordless drill and a short metal cleaning rod with a bronze brush on the end with a little steel wood wrapped around it and some abrasive paste or fine lapping compound smeared on it (don't go in so far you get up into the throat area).

If that does not work then it's time to look at whether it's over ported and the port needs to be bushed down in the barrel, or some other method is employed to cut back the gas flow. I know people seem to like them, but I don't like adjustable gas blocks. First off, your rifle has to be over ported for them to even be of use, and second, with an AR gas system with super high pressure and particulate matter shooting through there, there is no valve that can hold up well over time and not fail at some point (and they always seem to fail at the worst time).

If it's a carbine length gas system, that exacerbates the problem, epecially if the upper is over ported.

Robert Whitley
www.6mmAR.com

 

[chpprguy]

Re: DPMS LR308 failure to extract

Thank you all for the input.
I am going to inspect the chamber, maybe take an empty case with a loose neck and try to see how far the chamber pushes the bullet into the case, then compare that to the untouched milsurp and some fed redbox that I have....

I see the adjustable gas block and some other tune-up goodies in my future...

 

[rcw3]

Re: DPMS LR308 failure to extract

I would find it most surprising if your failure to extract issue is caused by the throating of your chamber. If your ammo is super high pressure stuff, a shorter throat can exacerbate difficult extraction, but it's not the cause of it. Some of that foreign ammo is not the best (to put it mildly).

 

[Casey Simpson]

Re: DPMS LR308 failure to extract

FGGM, Black Hills, etc.
Polish the chamber
Polish the bolt face including the exractor with microfibre and flitz while the bolt is chucked in a drill.
Do whatever Whitley and Zediker say.
While you're at it change the buffer, trigger and hammer springs to C S Superior Shooting System springs.

 

[AlterEgo]

Re: DPMS LR308 failure to extract

Lith is good stuff (for function not necessarily accuracy), I bet your M14 loves it.

Probably rough chamber, I'd check that avenue...

 

[Hellbender]

Re: DPMS LR308 failure to extract

Sounds like, in order..

1...Ammo problem (Soft cases and/or high pressure)

2...Possible rough or tight chamber

3...Overgassed for THIS ammo, may be fine for other.

If it was mine, I would polish the chamber (send back to DPMS for this, if you can't do it right) and put on an AGB anyway, as it can only help with any ammo, if done properly.

I agree with Robert on the AGB, but if you want to keep the stock barrel and the port is too large, you don't have a lot of choices.

 

[rcw3]

Re: DPMS LR308 failure to extract

Hellbender

We have bushed down gas ports that were too large in factory barrels (don't do it as a business but it is easy to do). Drill out a portion of the barrel where the existing port is located with a larger drill (like 5/32" or 3/16") but obviously do not go all the way down and into the bore, then put a metal plug made from something like drill rod in there (Loctited in) then re-drill it to a smaller diameter. The gas block over top of the plug will hold it in there and the smaller diameter of the hole through the plug will restrict the gas flow. The funny thing is you actually have to go a little smaller with the port in the bushing than you would normally go with a port in the barrel because for some reason the shorter bushing section allows more gas through it than would normally flow through a full length barrel port.

With some ports we went down .020" + to get what was needed (i.e. like from .093" to .070").

Robert Whitley
www.6mmAR.com

 

[sharpshtr66]

Re: DPMS LR308 failure to extract

had the same thing with a LR-308B, polished the chamber, no more rims riped off, just bent so put on JP adjustable gas block no problem since.

 

[Hellbender]

Re: DPMS LR308 failure to extract

Robert,

I've never heard of that, and I've messed with AR's for 30+ years.

Sent you a PM, also.

Thanks!

 

[TheRentman]

Re: DPMS LR308 failure to extract

If you look along the center of the barrel's bottom, you should see the stamp DPMS places on the barrels that indicates what ammo the weapon can utilize. Though .308 is stamped on the Lower, the bottom of the barrel will tell you for sure (some DPMS M-4s are stamped .223/5.56 on the Lower yet are only stamped .223 on the bottom of the barrel for example).

 

[sinister]

Re: DPMS LR308 failure to extract

Sir, by your photo it appears you have the 16-inch AP4 version with short gas tube.

I would recommend you first try a Tubb Carrier Weight System or an Extra Heavy (XH) heavy buffer sold by Clint Butler at www.heavybuffers.com ( http://www.heavybuffers.com/ar10carbine.html ) to delay unlocking and slow down your bolt carrier group cycling velocity.

Lithuanian 7.62 NATO (for M14s and other weapons they were given or bought from Uncle Sam) should function well. The problem with the short 7.62mm AP4 gas tube is the bolt carrier group will start to the rear while the cartridge body is still obturated and against the inside of the chamber.

The first thing to go will be your case rims -- the case isn't going anywhere while the bolt carrier group and extractor are trying to do their job.

 

[gunfrog]

Re: DPMS LR308 failure to extract

I had the same problems with my DPMS 16" 308 AP4, tried all the fixes: polish chamber, adj gas block, heavy buffer, AR carbine buffer tube and spring, moved gas port to mid length, and various combinations. Had some improvement but still had issues. I built another one with 762 chamber and it runs great.

Got me thinking so I finally checked the chamber in the problem gun and it closed on the no-go gauge. So I sent it off to very knowledgeable gun smith, to get the actual chamber dimensions and have it reset to 762. Still waiting on its return. Will advise.

Suggest you check your chamber with a set of gauges.

All the early DPMS guns of this type where made in 308, now DPMS only offers it in 762. They used to offer a 762 reamer to improve reliability with surplus ammo. They no longer offer the reamer, nor will they redo a 308 chamber to 762, based on a phon con I had with them about a year ago.