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AR-10 vs. DPMS Gas Tubes - FTF issue


Lancer1

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Hello all, great forum, I've spent many hours gleaning important information from the good folks on this site.  I have two LR-308 type builds. The issue I have is failure to feed with one of them.  I've read many of the posts concerning this issue, and wanted to run my thesis past your collective knowledge.  The offending culprit is put together with a Fulton Armory 20" stailess barrel, and a real DPMS upper receiver.  I'm not sure the BCG is traveling back far enough to eject, and then pick up the next round.  One issue is the poor performance of the DPMS 10 round mags.  The lips do not allow the rounds to peek up quite far enough, so I fixed that issue with an after market mag, higher lips, better feeding.  My issue now is a bit of a quandary; Although I purchase all DPMS compatible parts, I suspect that the FA barrel requires an AR-10 specific gas tube.  Look at the picture, both upper receivers have the same dimensions, but the gas tube does not protrude into the upper quite as far.  Both gas tubes are standard rifle lenght gas tubes (15-1/8") long.  I'm thinking the gas tube in the FA barrelled upper does not engage the Carrier Key quite enough, thus not delivering a proper pressure to blow the BCG all the way back.  The Armalite web site says their gas tube is 15-1/2" OAL, and I know a standard AR rifle length gas tube is 15-1/8".  In my picture, that looks just about the differnce in the protruding gas tubes.  What do you guys think?  Might I be on the right track?  Any inputs are welcome.  Thanks.  These rifles are deadly accurate, with both factory, and my handloaded ammo, albeit I have to shoot the one that won't feed as a "single shot" ::).  The upper on the bottom is the one in question.post-13708-0-56711300-1376371332_thumb.j

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Robocop, thanks for your reply.  Yes, I do see a difference in the distance, i.e, the shoulder were the gas block meets the larger diameter of the barrel on the Fulton Armory barrel is set farther foward by about 3/8" than the shoulder on the blue barrel (see picture).  Both gas blocks are bumped up against the shoulder (larger barrel diameter) and are clamped tight.  I just happen to notice the one gas tube did not enter the upper receiver as far on one, as it did the other.

post-13708-0-02794000-1376375092_thumb.j

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Like an old blue hair in a menthol smoke filled room, "Bingo!"

At least the fix is only a few bucks and is user end fixable. Count your blessings there. If you have any trouble finding an Armalite AR10 rifle length gas tube, give us a shout. You'd be surprised what we can find around here.

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I found and ordered an AR-10 Gas Tube for $12.  What is interesting is that Fulton Armory's barrel (gas block shoulder) is 3/8" farther down the barrel than many other DPMS products.  I have read so many accounts of feeding problems, with many possible causes.  There are really hundreds of theories as to why these guns don't feed correctly.  However, when doing a build yourself, you really have to look very precisely at the fitment of ALL of the parts.  You can't just assume each and every part will fit and function properly (from many different vendors), just because it is advertised as DPMS compatible.  Take the magazines, I have two actual DPMS mags that don't work.  I can clearly see that the primers of inserted rounds are lower than rounds placed in an after market mag (the lips have different angles).  The after market mag feeds correctly, the DPMS mags don't.  I have now ordered 4 different mags (from different manufacturers) searching for the best feeding mag.  I ordered two KAC mags, and they were $125 each. I notice that they are welded, not just bend (press) formed, so I'm assuming that's why they are so expensive.  Availability is scarce, but after much "googling", I have found; PMAGS, DPMS, KAC, PRO-Mag, and ASC.  I have not received all of them yet, so I can't testify as to the function of each of them.  I read one account of a fellow who purchased a completed DPMS factory LR-308 ($5500), and he has expierenced much feeding problems (he sent it back to DPMS twice, still does not work).  Other guys swear by their DPMS rifles, with no hitches whatsoever.  One theory that seems to make sense is the quality control has suffered, to meet the demands of the purchase frenzy.

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OK, I did not post here to be ridiculed. I was looking for constructive advice on why the gas port on a Fulton Armory barrel might be 3/8" farther forward, because it does not extend into the upper receiver as far as the other one (the other one works, feeds, etc..). Forgive my ingnorance, it was not my intention to irritate or amuse.

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Yes, the gas blocks are different.  However, both of them fit directly over the gas ports in both barrels.  The JP hand gard would be difficult to remove because it calls for thread lock, and I would have to heat it with a torch to take it off.  I suppose it might also be caused by the barrel nuts, if there is a difference in how long they are.  I didn't check that.  I hope the diamiter of the AR 10 gas tube is the same, so it will fit in the hole on the gas block.  If I can pin the AR 10 gas tube into the gas block, the added lenth of it should extend it to where it should be to meet the carrier key. 

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No worries Edgecrusher, I'm sorry I misinterpreted your coment.  From reading here, I realized that I didn't think of everything.  For example, it did not dawn on me until now that the barrel nuts could differ in length, or even the barrel extention (thanks survivalshop).  It's just a wake up call that when you build these rifles, you have to check everything (all components) to enusre they come together as the DPMS platform requires for everything to work.  Thanks to everyone for the inputs.

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Yes Imschur, I did see it.  The SASS stands for Semi Automatic Sniper System, right?  Does that have to do with the length of the gas tube?  I originally thought the the gas tubes were standard AR 15 rifle length gas tubes if used on DPMS type rifles.  I guess I don't know the specifics of the SASS gas blocks.  Do they have different dimensions?

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That's a good question , but I think there are no real spec's as you describe , only what could be milspec as far as Carbine or Rifle length gas tubes , now with the mid length . Not sure I have ever seen a spec on how far the gas tube penetrates the upper receiver .

 

I have to look to see if I have any thing in print on the subject.

Edited by survivalshop
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since you've already got the longer gas tube and it looks to be on the gun with the easily removable handguard, just remove the handguard, and slide the GB forward (after removing the gas tube roll pin) and try the new gas tube in there. You can verify that the gas key is not bottoming out on the end of the tube by measuring the back of the carrier, the location of the back of the carrier when the carrier is forward should not change when you install the longer tube.

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Jgun, both of the rifles in that picture have the standard, AR15 Rifle length gas tubes (15-1/8").  The bright stainless barrel's gas tube is the one I need to change because the standard gas tube does not reach far enough into the upper receiver, and I think that's why I'm not getting enough blowback on the bolt carrier.  I cannot push the gas block any farther reward, as it butts up against the shoulder on the barrel. Either the barrel extension on that barrel is longer, or the shoulder where the gas block butts up against it (and the gas port) is 3/8" farther forward towards the muzzel.  Whatever creates this 3/8 inch difference, I'm not sure.  Both gas tubes are the same length. This is why I have ordered an Armalite AR-10 gas tube, because it is 15-1/2" long, and I'm hoping it will fit in a standard gas block.  The extra 3/8" length of the Armalite rifle length gas tube should make up the difference, and extend into the upper receiver to where it should be.  The bolt carrier closes into battery in both uppers, so if the Armalite gas tube fits into the upper as it should, both of my gas tubes will extend into the receivers to the same length.  Thus, I should get a full charge of gas, pushing my BCG back far enough.  I have even purchased a JP adjustable gas block so I can "turn it down" if I'm getting over pressure.  What do you think?

Edited by Lancer1
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I apologize for not seeing this sooner. Everything you are saying makes perfect sense, and was what I was suggesting, that you just try the AR 10 tube (that you said you had already ordered). the only thing I was cautioning you about was to verify that the gas key was not bottoming out on the longer gas tube before the bolt was closing completely, (it shouldn't). That was why I was suggesting that you close the bolt without a gas tube installed and measure from the back of the carrier. The measurement should not change when you install the AR 10 gas tube. if it doesn't, you shouls be GTG. Be sure to let us know how it cycleswith the longer tube installed.

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Jgun, I'll take your advise about making sure the gas key doesn't bottom out with the longer gas tube in there.  I should get the AR-10 gas tube tomorrow in the mail.  I've learned alot about .308 magazines trying to fix this.  The actual DPMS 10 mags suck!  PRO Mag 10's work good, ASC mags work good and these are relatively inexpensive.  The really nice ones are the actual KAC, they are beautifully made (tig welded), but cost $124 (the only KAC's I could find).  Magpul 20 round (modified to block them to 10 because I'm in California) seem good, I will shoot them this week.  I will post the results of the AR 10 gas tube when I get my hands on it.  Thanks for your input.

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Yes DPMS monkeyed with the barrels and gas blocks on the SASS rifles.

Imschur is correct .....sass gas blocks are different dimensions.....I had a super sass barrel and had to get the sass gas block for it jse had them for like 20 bills . Its the height of where the gas hole is for the gas tube.

you may have a torque (bend) issue if you use a regular gas block on a sass barrel....hmmm maybe length too :) Wash

also dpms had a bad run of mags a while back

Edited by washguy
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