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Discussion Starter · #1 · (Edited)
Hey all, I have a 300 AAC Blackout AR pistol I built a few months ago:
http://www.xdtalk.com/forums/ar-talk/254438-build-number-2-300-aac-blackout-8-pistol.html
Anyways, it has been giving me nothing but grief. I tried 2 different bullets each with 2 different powders, as well as some remanufactured ammo from a place 20 minutes from me. I got one set of my reloads, as well as the remanufactured ammo working from a benchrest.

However, when I progressed from benchrest to firing upright, it quit working again, at least sort of. Everytime I fired it like a pistol (aka no support other than two hands on it, it would eject the spent case, but not chamber a new round. I individually tested each of the few mags I use with this gun by single firing a round in the mag, and the mag locked back each time, so the mags should be good. I also inspected the chamber for any irregularities and didn't find or feel any.

I figured out that if I fired it with it shouldered (which doesn't feel to good since it jams the tube into my AC joint), it cycles sometimes.

I guessed that it was overgassed so I bought an H2 buffer knowing I could make an H buffer out of that and my carbine buffer. I took it out two weeks ago with the homemade H buffer. Also put extra oil on the BCG, since I know that can be a problem for some ARs. Suddenly the same exact handload that would not cycle with a carbine buffer would cycle every time, but not lock back on empty (with the same mags that I individually tested). However, when I shot some more of the remanufactured ammo, it would no longer cycle.

Confused as could be, I went and bought some 125 gr OTM Remington Premier Match ammo, I took apart and reassembled the gas block and gas tube, and went out today with the H2 reassembled and the carbine buffer. With the H2 buffer in it, I fired 5 rounds with it shouldered. It cycled twice, I had to charge it twice because it ejected but didn't strip the next round, and it didn't lock back on empty (same mags that were test and worked, then stopped working when I added oil). I fired it like a pistol 3 times, and it only ejected the spent case, but didn't strip the next round (didn't reach the bottom of the 5 rounds loaded cause I got frustrated. I switched to the carbine again, and fired 5 rounds from my shoulder. 3 ejected and didn't strip the next round, and 1 started to, but didn't finish. It also didn't lock back on empty.

So now I am lost. I am assuming my BCG is sensitive to too much oil, as it stopped locking back, so next time I won't oil it as much. I will try the H1 buffer with the match ammo shouldered.

Any other ideas? Why won't it work like a pistol? I want to buy a Sig brace since it seems to work better supported, but I feel as if I might as well set up a trust and SBR it if I am going to do that, and I don't want to waste money on an SBR that won't shoot.

I am open to any and all suggestions, as I am at my wits end on this.
 

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Any kind of pinch in the gas tube? With it ejecting but not picking up the next round it sounds like not enough gas. Just enough to eject but not enough to fully cycle the bolt rearward to pick up the next one. Just my thoughts. I'm new to the AR game though.
 

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Discussion Starter · #3 ·
I haven't seen anything unusual in the gas tube, but good suggestion. I also forgot to mention above that I also took the gas system apart and reassembled it prior to today.
 

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Three things occur to me to check...
1) Gas Tube seating at the Upper and the position of the Gas Block on the barrel...it seems to me the ports may be restricted by not being completely inline..it happens a lot (don't ask me how I know that! )

2) Your Buffer Assembly--is the buffer too heavy? Is the Buffer Spring the correct one?

3) Is your BCG assembled and staked correctly? Properly lubed? Rearward momentum could be inhibited if not...

Let us know whatcha figure out
 

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Where are the spent brass ejecting, 2o'clock? 3?

Since you built the entire upper, is the gas tube entering the upper straight and true without touching or being influenced by the barrel nut?

Have you checked the barrel gas port for burrs or other obstructions?

Is the gas block on tight and properly aligned with the gas port?
 

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what is the spacing from the back of the gas block journal on the barrel to the edge of the hole?

is that distance the same on your gasblock?

I know a lot of people make the mistake of driving their gas blocks tight against the shoulder of the journal on DCM spec barrels. this is actually wrong as the handguard plate give the correct gap for optimal gas port alignment. otherwise you will have an undergassed system.
 

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Sounds like you're having an undergas issue, not an overgas. Usually culprits, check the gas tube for kinks, leaks etc, and make sure the fitting into the gas block isn't pulling out. And perhaps too heavy of a buffer. Gas keys okay?
 

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Discussion Starter · #8 ·
Three things occur to me to check...
1) Gas Tube seating at the Upper and the position of the Gas Block on the barrel...it seems to me the ports may be restricted by not being completely inline..it happens a lot (don't ask me how I know that! )
I took the Gas Block off and re-attached it before today, but I am probably going to re-check again this afternoon
2) Your Buffer Assembly--is the buffer too heavy? Is the Buffer Spring the correct one?
I have tried a carbine, an H, and an H2 buffer. None have worked perfectly. I just took out the spring and measured it. Untensioned, it measured just a hair under 10.5 inches.
3) Is your BCG assembled and staked correctly? Properly lubed? Rearward momentum could be inhibited if not...
Learning how to disassemble and reassemble my BCG may be the next task on my list.
Let us know whatcha figure out
Replies in Red
 

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Discussion Starter · #9 ·
Where are the spent brass ejecting, 2o'clock? 3?
I had the brass catcher on today because it was crowded and no walls between people, but with the H1 buffer last time it was landing 5-6 feet away between 3 and 4:30.
Since you built the entire upper, is the gas tube entering the upper straight and true without touching or being influenced by the barrel nut?
I haven't noticed anything unusual. I will check again tonight when I take it all apart.
Have you checked the barrel gas port for burrs or other obstructions?
I will check tonight.
Is the gas block on tight and properly aligned with the gas port?
As far as I am aware
 

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Discussion Starter · #10 ·
what is the spacing from the back of the gas block journal on the barrel to the edge of the hole?

is that distance the same on your gasblock?

I know a lot of people make the mistake of driving their gas blocks tight against the shoulder of the journal on DCM spec barrels. this is actually wrong as the handguard plate give the correct gap for optimal gas port alignment. otherwise you will have an undergassed system.
I left a small gap between the gas block and the end of the shoulder of the barrel, but I will go measure the two distances to be sure I have it space properly.
 

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Discussion Starter · #11 ·
Ok, so I took the the whole thing apart and reassembled it. Barrel is good, free float is lined up with gas tube, gas block is lined up with gas port. However, when I was playing with the gas tube, it was loose. I looked closer, and the gas tube roll pin was gone. I know it WAS there, because I fought a war getting it in. I have zero idea what happened to it, but it is gone.

Luckily, I had a extra gas tube and roll pin I accidently ordered, so I just put the new gas tube and roll pin in.

If it doesn't rain tomorrow I will take her for another test run, if not it will be Friday.
 

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Your under gassed. First lighten it up the buffer not heavier.

Then check gas key
Gas ring on bolt the gaps aren't lined up.
Gas tube perfectly straight and centered inside upper? Remove Bcg/handle look with flashlight.

Front post centered on gas hole? Left/right and did you make the gap so centered front rear?

If none of those solve take apart check gas tube block and hole in barrel for blockage reassemble see if it works.

If it doesn't swap out lower and or upper or Bcg and using process of elimination figure out what's out of spec.

Safety is a momentary choice. Prior performance and certification are completely irrelevant.
 

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Ok, so I took the the whole thing apart and reassembled it. Barrel is good, free float is lined up with gas tube, gas block is lined up with gas port. However, when I was playing with the gas tube, it was loose. I looked closer, and the gas tube roll pin was gone. I know it WAS there, because I fought a war getting it in. I have zero idea what happened to it, but it is gone.

Luckily, I had a extra gas tube and roll pin I accidently ordered, so I just put the new gas tube and roll pin in.

If it doesn't rain tomorrow I will take her for another test run, if not it will be Friday.
Lol nvm

Safety is a momentary choice. Prior performance and certification are completely irrelevant.
 

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Discussion Starter · #14 ·
Lol nvm

Safety is a momentary choice. Prior performance and certification are completely irrelevant.
Your first statement was correct lol, no way enough gas was getting through a misaligned tube. just a little late
 

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Ever figure out what the problem was?
 

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Discussion Starter · #19 ·
Ever figure out what the problem was?
I took it apart and found that the gas tube roll pin was gone. I know for a fact that I installed it, so I don't know if it broke or wiggled out or what, but I put a new gas tube and roll pin in and an taking it out this afternoon. Hopefully that was the problem, since workout it the gas tube was loose, therefore not aligned with the port.
 

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Cool well I hope all works well.
 
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