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Posted: 11/8/2014 4:35:55 PM EDT
[Last Edit: 11/27/2014 3:06:51 PM EDT by Blackhawk5]
This is my first short barreled AR build.

Shot it for the first time today to zero the optic. About 60% of the time, it would jam trying to chamber the next round. The bolt does not lock back after the last round everytime. Showing significant brass markings on the brass deflector and more dents in the brass than usual. When I disassembled the gun, there looks to be unburnt powder in the bolt and chamber.

Ammo is PMC .223 55gr FMJ

-Spikes upper
-AAC 51T blackout Muzzle Brake
-Rainier 10.5" 5.56 barrel
-Geissele super gas block .750"
-Spikes melonite gas tube (carbine)
-PSA BCG
-Armalite stock buffer and spring

Magazine is HK polymer mags, ran 100% through my 16" AR, so I know they are not the problem.
Lower was tested on a 16" upper and had no issue.



What would be the best way to fix this problem?
It seems like changing to a lighter buffer and spring would fix this? I'm wondering what type of setup you guys have with this barrel length to run smoothly.


Link Posted: 11/8/2014 5:13:39 PM EDT
[Last Edit: 11/8/2014 5:20:43 PM EDT by BlackRifleJuju]
What is the weight of the "standard" buffer? You may actually need something heavier.
I have a 10.5" AR that runs fine with the stock components I got with my PSA build kit, but I have a buddy with a 7.5" AR and he actually needed a heavier buffer.
His was malfunctioning the same as yours. After we put the heavier buffer in it ran like a top.
Link Posted: 11/8/2014 5:36:55 PM EDT
I would also recommend a heavier buffer. You are probably overgassed.
Link Posted: 11/8/2014 9:01:40 PM EDT
Link Posted: 11/8/2014 10:35:13 PM EDT
[Last Edit: 11/8/2014 10:35:24 PM EDT by Blackhawk5]
Every round ejects, but not all rounds are able to chamber...sometimes left hanging like in those pictures or no round in the chamber at all...so this is an overgassing problem?

Would something like this solve the problem? http://www.bravocompanyusa.com/ST-T2-Heavy-Buffer-by-Spikes-Tactical-p/st-t2.htm

I reload, but I will try some hotter ammo like federal 5.56 through it, as I am using factory ammo as a baseline.
Link Posted: 11/9/2014 3:03:55 AM EDT
Link Posted: 11/9/2014 1:09:02 PM EDT
[Last Edit: 11/9/2014 1:11:05 PM EDT by Blackhawk5]
See some slight wear on the rim of the cartridge from the extractor, but not enough to dent it in a major way or deform the rim.

How would be a way to check for proper alignment of the gas block and potential leaks short of firing it?

The Bolt carrier fits the gas tube inside the upper with no interference.
Link Posted: 11/9/2014 2:28:14 PM EDT
Are there any powder sneezes around the gas block or anywhere?

And I assume you did the last round hold open test before posting, yes?
Link Posted: 11/9/2014 2:57:29 PM EDT
How is that an overgas or too light a buffer problem? Maybe I'm looking at it wrong but seems like under gassed and short stroking to me which would be lighter buffer and/or spring while making sure the gas block isn't choking things via misalignment or not opened enough for the adjustable. Getting just enough clearance to eject but not quite enough to get all the way behind the next round and strip it from the mag properly, it appears to be attempting to strip it by grabbing/scraping it out of the mag along the case wall.
Link Posted: 11/9/2014 5:00:13 PM EDT
[Last Edit: 11/9/2014 5:02:54 PM EDT by Blackhawk5]
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By Justintime2:
How is that an overgas or too light a buffer problem? Maybe I'm looking at it wrong but seems like under gassed and short stroking to me which would be lighter buffer and/or spring while making sure the gas block isn't choking things via misalignment or not opened enough for the adjustable. Getting just enough clearance to eject but not quite enough to get all the way behind the next round and strip it from the mag properly, it appears to be attempting to strip it by grabbing/scraping it out of the mag along the case wall.
View Quote

I thought so at first, but every round I fired was able to eject, and made a visible marking on the brass deflector. Looking at some videos of short stroking, the ejected rounds did not touch that far back and you would still have some ejected rounds in the chamber or partially sometimes, which i never seen. The problems I encountered were only when cycling the next round most of the time and locking back on an empty magazine.

I weighed my buffer to be exactly 3.0 oz
Also I had another buffer laying around made by PSA and it was also 3.0 oz.
Bolt holds open after last round when I manually do it, tested with HK, PMAG, and GI aluminum mags.
Link Posted: 11/9/2014 5:35:47 PM EDT
Link Posted: 11/10/2014 11:29:02 PM EDT
[Last Edit: 11/10/2014 11:32:52 PM EDT by Blackhawk5]
The signs are pointing to that it's undergassed. The brass ejects at the 3-5 o'clock position and I don't see any excessive wear on the cases, besides when the bolt gets stuck trying to cycle the next round (as in the pictures in op). Plus I didn't feel any excessive recoil or violent action, but the muzzle brake may have a play in that.
Pictures that may help diagnose if I could be wrong.

yellow stuff is powder or brass??




(open direct link for larger res)



I will check the gas tube/blocks for any leaks tomorrow, but the barrel is dimpled and set screws located on the bottom of the gas block, there wasn't a lot of freedom of movement on the gas block when the screw hit the dimple.
The next time I head to the range, i'll try higher pressure ammo. I'll also switch it out with a PSA buffer and spring, which has the same overall weight as my Armalite but looks to have 1 less spring coil.
Short of drilling a hole on my barrel, is there a buffer that can fix this that people have had luck with?
Link Posted: 11/11/2014 12:48:01 AM EDT
[Last Edit: 11/11/2014 12:49:52 AM EDT by Dano523]
Link Posted: 11/11/2014 9:47:24 PM EDT
[Last Edit: 11/11/2014 9:48:49 PM EDT by Blackhawk5]
The best I can get with my phone, extracted surface on the top


to me, it doesn't look too out of the ordinary
The dings near the neck of the case seem more concerning, although that could be from bolt hitting the case as it tries to cycle like in OP
Link Posted: 11/11/2014 11:59:14 PM EDT
Link Posted: 11/12/2014 5:20:18 AM EDT
[Last Edit: 11/12/2014 5:34:32 AM EDT by ellementz]
Think I heard of someone having the same problems with the Rainier 10.5" 5.56 barrel. I think head spacing and the feed ramps were the problem for him.




Do your feed ramps look like the picture on the left?
Link Posted: 11/12/2014 8:22:20 AM EDT

All of my parts, including the barrel, were bought new. Last week was the first time I fired it. Also fired a few hundred rounds with the lower on a 16" upper to test for function and had no problem.

Someone suggested polishing the chamber, could that be an issue?

I'll look into the feed ramps tonight.
Link Posted: 11/12/2014 5:49:06 PM EDT
Ramp on my pistol with PSA upper looks like the first pic, and I was having same issue every 2-4 rounds.
Tried different buffer without avail, but what got it working was PMAG G2.
The PSA aluminum mag worked just fine in my rifle so didn't think of trying PMAG at first.
Link Posted: 11/12/2014 9:41:22 PM EDT
Not sure exactly what to look out for, but here's what mine looks like


Link Posted: 11/12/2014 11:59:03 PM EDT
Link Posted: 11/21/2014 10:54:42 PM EDT
Got it to work flawlessly with various mags, just needed to have a little more pressure of 5.56. Even put my spikes T2 buffer in with no problems. Tested it with xm193 and my handloads just fine. Just can't use PMC or steel cased .223, which is the other ammo i tested. Made sure to run it wet. No noticeable unburnt power residue either.
Link Posted: 11/22/2014 6:00:43 AM EDT
Did you try wolf gold in it?
Link Posted: 11/22/2014 9:31:07 AM EDT
Dont own wolf gold or know anybody that does. The steel cased was tula .223. Just gonna stick to 5.56 on it for now. My last resort was drilling a larger hole in the gas port, but that is unlikely now that seeing 5.56 run through it is pretty significant pressure.
Link Posted: 11/22/2014 10:23:48 AM EDT
The last few barrels I bought through Rainier have had very tight chambers. (.308 and .300 BlackOut)

Rounds that would fully chamber in other barrels, wouldn't do so these barrels. I'd have to mortar the rifle to get the round out.

Easy enough fix, for me.

I just polished the chambers with very, very fine emery pads, and polishing compound applied to a bore swab.

Just be sure to only do the chamber, and avoid the lands/grooves.

Link Posted: 11/22/2014 1:09:22 PM EDT
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By Blackhawk5:
Got it to work flawlessly with various mags, just needed to have a little more pressure of 5.56. Even put my spikes T2 buffer in with no problems. Tested it with xm193 and my handloads just fine. Just can't use PMC or steel cased .223, which is the other ammo i tested. Made sure to run it wet. No noticeable unburnt power residue either.
View Quote


Thanks for the update, OP.
So, what we learned is that it was a relatively undergassed situation (short stroking) due to relatively low pressure of the initial ammo used.
I think that 98% of these problems can be fixed by following the instructions in Sully's and Dano's posts. And BR JuJu had the bolt open test nailed.
I'm just going to copy and paste them in the future and take credit.
Link Posted: 11/27/2014 3:21:29 PM EDT
My 10.5" Rainier barrel was undergassed with .223 and my Law Tactical folding kit (buffer extension adds weight). Enlarged the gas port .005" to .081" and it ran flawlessly. Now with the folder removed and a standard carbine buffer, I'm using a Micro MOA adjustable gas block with a .078" port and it also runs flawlessly. I wouldn't be scared of opening the gas port a bit if you need to, it's not a big deal if you have a drill press or mill (or steady hands).
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