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Problem with DPMS 18" fluted bull barrel upper assembly


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#1 biozzcx

 
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Posted 01 December 2010 - 02:55 AM

I'm new to 308 AR and this forum.  As the title indicates, I have a problem with a complete upper assembly build on a DPMS lower.  I use carbine buffer tube, DPMS 308 lower kit, AR 15 carbine length buffer, and Tubb Superior shooting buffer spring (for 308).

I was following DPMS's instruction for break-in the barrel.  The ammo used was Federal 150 gr soft point from Walmart. I loaded one round to Pmag (Magpul 20 rounder), fired it.  Cleaned the barrel; loaded another round to mag, chambered and fired it, no problem.  Cleaned barrel again, loaded 2 rounds in the mag, the first one fired, pulled the trigger again, nothing happened.  Now, with a live round in the chamber, the bolt was not completely closed, I couldn't remove the round from chamber, the bolt was locked.  Luckily, I was able to open the bolt with a padded screwdriver and removed the live round.  I then inspected the feeding ramp, it was rough.

My best guesses are: 1. the buffer spring is too strong that the bolt was not fully pushed back to correctly chamber a following round; 2. the rough feeding ramp dragged the following round and in combination to possible problem 1; 3. resulted from soft point ammo; or 4. had something to do with headspace.

I change to regular AR15 carbine buffer spring, re-cleaned and re-oiled the bolt assembly, polished feed ramp with dremel polishing wheel.  This time I used surplus ball ammo for a test.  It was better, one of five round got stuck with the bolt not complete close, forward assist didn't help.  It was not as tight as before.  I could eject the round forcefully with the charging handle.

My thoughts are that the bolt assembly is still tight, and feeding ramp still needs some smoothing and polishing.  I assume that the upper assembly should be headspace at DPMS and test fired.  Before I try shooting again, I would like to hear from experts here so I could avoid incidence or damage to me/my rifle.  I appreciate your inputs.

Paul

#2 imschur

 
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Posted 01 December 2010 - 03:24 AM

first off you need a 308 buffer. The Tubb spring should be good. These weapons need a lot of lube when new. Mobil 1 or equivalent

Theres more info on buffers here


#3 biozzcx

 
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Posted 01 December 2010 - 03:27 AM

Just read the post in technical section of DPMS LR-308, I realized that I used a WRONG buffer! My bad! Please forgive a newbie. 

Paul

#4 TacBlade

 
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Posted 01 December 2010 - 03:28 AM

I am not an expert on this, as I just finished my first build.  But, I believe you are having the same problem I was having.  I would put the .308 spring back in, as it is more than likely not the problem.  I found that when this happened to me, I had to really clean the chamber up good or even give it a slight polish.  I did this a couple of times and after about 50 rounds, I had no problems. It seemed as though the initial firing of the rifle really dumped alot of crap into the chamber and made it really tight.  Also, really oil the bolt assembly up good before you fire it.  This may not have to be done so much down the road, but the first 20-40 rounds it seems to help alot to break it in.  Just my thought and what worked for me.

#5 TacBlade

 
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Posted 01 December 2010 - 03:30 AM

imschur posted while I was typing mine and he hit the nail on the head!!!  I missed the part where you said you used a AR15 Carbine buffer!!!  Change that out and lube it up and you should be good to go!!!

#6 YWHIC

 
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Posted 01 December 2010 - 03:42 AM

Thanks for the tips.. I got some Miltec TW25B on the 'rails' and will be using some Mobil 1 oil on mine also prior to firing for the first time.. I thought it was 20 rounds then clean.. then 1 round and clean routine?? didn't read my booklet yet...

But yes.. ALOT of people have used the std AR15 buffer & spring and had the same - results.. keep the 308 one in an you should be G-t-G..


Al

#7 biozzcx

 
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Posted 01 December 2010 - 03:46 AM

Thank you guys! I will get the right buffer soon.

Paul

#8 imschur

 
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Posted 01 December 2010 - 03:50 AM

Glad to help Paul.

You insprired me to create a new sticky for the faq section


#9 imschur

 
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Posted 01 December 2010 - 03:52 AM

Remember run them wet until they are broken in. More lube than you would an AR-15




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