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Hexahydro
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Posted: 11/18/2009 2:27:17 AM
[Last Edit: 11/18/2009 2:28:28 AM by Hexahydro]
The Parts:

Standard H buffer and buffer spring inside a Bushmaster tube
Young Chrome Full Auto BCG (minus the bolt)
Superior Barrels 18" Terebratus barrel with matching bolt
(.223 Wylde Chamber, 1:8 male polygon rifling, M4 feed ramps, made from stainless and treated with hardblue)
YHM low pro gas block
Gas Tube from Bravo Company
Essential Arms lower with LPK from M and A Parts



The Problem:

I assembled it myself, completed a functions check, then took it to the range. I had basically put together a bolt action AR,
after every shot it would short stroke, sometimes it wouldn't extract the spent brass from chamber, other times it would go
as far as ejecting the spent brass and resetting the sear but not feeding a new round. After 20 rounds or so it became
harder and harder to cycle by hand to the point of (after clearing it) banging the stock against a table while a buddy pulled
the charging handle back. At that point I stopped and took it home.

After disassembling it I noticed the M4 feed ramps of upper receiver and barrel extension did not line up properly, I assume
I over torqued it putting the barrel nut on. They were misaligned enough to be noticeable by eye, if I had to guess I would say it
was off by about 3 degrees. I placed the upper in a vise and an armorers wrench on the flash hider to turn the barrel and line
up the feed ramps properly. Hoping that was the cause I went back to the range. It wasn't the short stroking continued.

This time at the range it was still short stroking but it was no longer difficult to cycle by hand, assuming it was from a lack of
pressure I put my suppressor on it and it worked as it should for over 100 rounds. When I took off the suppressor it was still
short stroking and I noticed a large build up of carbon on the barrel just forward of the gas block. I am using the correct diameter
gas block for my barrel, and the correct length gas tube, there is a roll pin in the block holding the tube in and I tightened the 2 set
screws as tight as I could, and used blue loctite, and the gas block is properly aligned with the barrel as best as I can tell.

What is the most likely cause of the short stroking? I noticed most other gas blocks do not have a hole all the way through them
like the YHM does, could this be the problem? Or is it likely caused by something I did during assembly? I could switch to a lighter
bolt carrier but I don't think I should have so much (or any) gas coming out of the gas block.
Thanks

ETA- I'll post pics of the carbon on the barrel when I can
climbr1
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Posted: 11/18/2009 8:40:40 PM
The gas block is blocked at one end so the open end isn't an issue. When you say you tightenend the set screws as hard as you can with what a Allen wrench, bit driver, or socket adapter? Sounds like a gap problem in the GB. Did it slide on easily or require tapping? Did you use the pin that came with the gas tube? Just thinking undersized roll pin, wobbled out hole from torquing the flash hider?
LiveToDrive76
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Posted: 11/18/2009 9:21:43 PM
Is it possible that the YHM gas block was installed backward and the holes are not lining up?
Hexahydro
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Posted: 11/18/2009 9:36:11 PM
Originally Posted By climbr1:
The gas block is blocked at one end so the open end isn't an issue. When you say you tightenend the set screws as hard as you can with what a Allen wrench, bit driver, or socket adapter? Sounds like a gap problem in the GB. Did it slide on easily or require tapping? Did you use the pin that came with the gas tube? Just thinking undersized roll pin, wobbled out hole from torquing the flash hider?


I assume you mean the gas tube is blocked at one end so the open end of the gas block is not an issue.
I tightened the set screws with an allen wrench, but it is a smal size so it's possible I didn't apply enough
torque to the small allen wrench. The gas block required a little bit of tapping to get it in place. I bought the
gas block and gas tube from 2 different companies, neither supplied a roll pin, so I purchased a roll pin from
Bravo Company. My rails cover my gas block and I may have to take them off to see the roll pin but I will
check to make sure it is still in place.

Thanks
Hexahydro
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Posted: 11/18/2009 9:41:17 PM
Originally Posted By LiveToDrive76:
Is it possible that the YHM gas block was installed backward and the holes are not lining up?


Anythings possible. When I put together the barrel, gas block and gas tube I plugged the chamber
blew air into the muzzle and could feel it coming out of the gas tube. If it was backwards wouldn't
the rifle not cycle at all? It has to be getting some gas to at least short stroke, I think.
LiveToDrive76
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Posted: 11/18/2009 9:50:56 PM
Is the roll pin closer to the muzzle or the handguard?
climbr1
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Posted: 11/18/2009 10:06:05 PM
Sounds like you got quality stuff from BCM so speculating any further takes me out of my lane. Sorry bro
Hexahydro
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Posted: 11/18/2009 10:52:24 PM
Originally Posted By LiveToDrive76:
Is the roll pin closer to the muzzle or the handguard?


I'm at work so I won't be able to check for a few hours, but just to save myself some
embarassment, which side is it supposed to be on?
LiveToDrive76
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Posted: 11/19/2009 12:17:21 AM
Muzzle FTW!
Signal-O
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Posted: 11/19/2009 12:59:00 AM
Sounds like you have two separate problems. The short stroking is definitely a gas leak somewhere. You just have to find it and correct it. My guess would be the YHM gas block or the gas tube not fitting or installed correctly. The problem I'd be more worried about is the bolt carrier assembly tightening up like it did. I don't like the idea that the barrel extension or the barrel itself may have shifted in the upper receiver somehow, not allowing the bolt head to allign with the barrel extension and release. Especially after you had the bolt matched to the barrel. If it initially cycled smoothly before it tightened up I would probably have a professional take a look at it before it locks up again. Just my 2 cents. Post some pics if you can. Good luck.
Hexahydro
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Posted: 11/19/2009 8:26:22 PM
Thanks for evertbody's help. I haven't had a chance to even look at it yet, but I will post some pics this weekend.
Hexahydro
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Posted: 11/20/2009 11:14:45 PM
Originally Posted By Hexahydro:
Originally Posted By LiveToDrive76:
Is the roll pin closer to the muzzle or the handguard?


I'm at work so I won't be able to check for a few hours, but just to save myself some
embarassment, which side is it supposed to be on?


I finally checked, the roll pin is still in place and the gas block is installed facing the right way.

Heres a pic of carbon left behind from the gas leak. The barrel is Duracoated green, I wiped off a strip to show contrast.
When I painted the barrel I taped off everything and there was no paint near the gas port or where the block goes, so it
shouldn't be a contributing factor.

climbr1
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Posted: 11/21/2009 1:40:43 AM
Keep me updated, I just assembled a new middy with a bcm barrel a yhm low pro block and I haven't fired it yet. But the block doesn't look right for some reason on mine. I'm suspicious
eric10mm
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Posted: 11/30/2009 5:11:57 PM
Have you ruled out that the gas tube might be blocked (from the factory) yet?
“When the people fear their government, there is tyranny; when the government fears the people, there is liberty.” Thomas Jefferson
Hexahydro
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Posted: 11/30/2009 8:04:46 PM
Nope, but I tore it all apart (somewhat literally) and I'll be getting a new gas block and tube
Billetman
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Posted: 11/30/2009 8:49:58 PM
What ammo are you using?
Did you try it with a different BCA?
DId you check the carrier key for blockage?
Are you using normal gas rings or 1 pc gas ring?
Lots o lube on everything?
How much slop in the gas block to gas tube fit?


B-man
Hexahydro
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Posted: 11/30/2009 9:19:08 PM
Ammo: Federal XM193, M855, Wolf, and Black Hills factory seconds, all had the same problem
BCA: Nope, but I should have. I did try a different bolt, and that set up worked perfectly in a different rifle.
Carrier Key: Not then, but I just looked and I can't see a blockage.
Gas Rings: Normal
Lube: Oh yea, I used alot and then some more
Slop in fit: None once the roll pin was in, before that I could not easily move it with bare hands.
Billetman
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Posted: 11/30/2009 9:47:46 PM
Originally Posted By Hexahydro:
Ammo: Federal XM193, M855, Wolf, and Black Hills factory seconds, all had the same problem
BCA: Nope, but I should have. I did try a different bolt, and that set up worked perfectly in a different rifle.
Carrier Key: Not then, but I just looked and I can't see a blockage.
Gas Rings: Normal
Lube: Oh yea, I used alot and then some more
Slop in fit: None once the roll pin was in, before that I could not easily move it with bare hands.


Make sure you can blow through the key like the tube. Believe it or not I've had a primer fly into the key hole and plug it up. It was a bitch figuring that one out!
The other thing you might want to check is for a sheared bolt on the carrier key. Buddy had it happen on a stage at a local match. Just started short stroking and he couldn't figure it out. Somebody told him to check and sure shit one was sheared in half.
Your pic of the gas flow ahead of the block looks excessive though for so few rounds.
Try measuring the barrel OD and the gas block ID. A lot of calipers are not exact but it might give you an idea.
I ordered in 4 PRI lo-pro .936 gas blocks earlier this year and One of them measured .942". WOOPS! Must have slipped by the QC dept.

B-Man