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Posted: 2/6/2008 10:08:53 AM EDT
I completed my first AR build a couple of weeks ago.  I purchased a 16" mid-length carbine with collapsible stock kit from M&A Parts and a stripped DPMS lower from Coleman Tyler.  I planned on installing a Yankee Hill free float handguard with their low profile Specter gasblock.  When I ordered my kit from M&A, I told them that I did not need the standard handguard or front sight/ gas block assembly.  Due to my request, the kit I received included an un-barreled upper and separate barrel and all other required parts.  M&A Parts states that all stripped barrels have been properly head spaced by LMT who finishes their barrels and provides their uppers.  I had no problems with the build, it was completed in a couple of evenings.

From the very first test firing session, my AR has pretty much acted as a bolt action for the approximately 100 rounds I have through it.  I have had to manually cycle the bolt to eject and load rounds. Rounds feed and eject without issue when cycled by hand.  The action of the bolt (with standard gas rings)and carrier seems smooth. On occasion, the shells eject but new rounds are not chambered nor will the bolt hold open on an empty magazine.  I can hear the sound of the spring in the buffer tube when firing.  The stroke on the bolt seems to be only about one inch or so.  Most of the ammo I have used has been cheaper brands such as PRVI 5.56, Remington UMC .223, and Silver Bear .223 (55 and 62 grains).   I have not yet used any bore cleaner on it, only CLP but I have tried polishing the chamber with the 2000 grit sand paper as suggested in the tacked Short Stroking thread.  I disassembled it once to make sure the gas block was properly lined up and it did appear to be.  Recently, I noticed some carbon deposits on the gas tube just behind the gas block as shown in the photo below.


I decided to strip my AR down to take some measurements and photos to post here so that I may ask for the assistance of those more knowledgeable than me in diagnosing my problem.

With the forearm removed, you can see the carbon deposits on the gas tube that extend less than 2 inches from the block.

Are these deposits on the gas tube typical or do they indicate gas is leaking from the system?  If gas is leaking from the system, does this appear to be a gas tube or gas block issue?  A low profile gas block is required since I have Yankee Hill's Specter length FF forearm. The tube seems to fit snug in the block and requires some force to remove it.  Dimensions of the gas system parts are indicated later in the post.

The photos below show the carbon pattern on the gas tube.

I also noticed some moisture on the barrel near the gas block.  It smelled like CLP was either blown out the gas port or residule from prior cleaning.  The block was tightened down.  I was worried I would bend or break my hex key. There did not appear to be any carbon deposits on the inside of the block but there were some chunks between the upper and lower sections of the gas block.


Based on the carbon ring left around the barrel port, the block seemed to be properly located.  

The one thing I am concerned about is the diameter of the gas port in the barrel.  The 0.065" that I measured seemed like it might be a bit small but I do not know what the standard is for a 16" mid-length barrel with a 0.75" diameter.  

Dimensions of the gas system are as follows:

  • Gas tube ID @ key = 0.1225"

  • Gas tube OD @ key (mushroom)= 0.179"

  • Key ID = 0.183"

Dimension of the system at the barrel end are as shown in the following photos:

I feel I have done what I can with all of the great info I have mooched from past threads.  I am to the point now that I need help from a higher power, the knowledgeable members of AR15.com.

I hope I have included enough info to help diagnose the problem.  I would like to thank you all in advance for your kind assistance.
Link Posted: 2/6/2008 7:36:20 PM EDT
Link Posted: 2/9/2008 10:42:38 AM EDT


other than the comments above, i don't see anything really amiss with your setup.  tell us a little more about your bolt, carrier, key, rings, buffer, spring, and stock.

i would like to make a comment on your post though:  it should be tacked and used as an example of how to do everything RIGHT when making a post in the troubleshooting section of ARFCOM.  kudos to you for bucking the usual "i built me a AR and it don't shoot right -- what do i do now?" method of asking for assistance seen here all too often.

congratulations on joining ARFCOM, you are already an asset to the operation.  my best wishes for a speedy solution to your short-stroking problem.

Link Posted: 2/10/2008 4:35:35 PM EDT
Tell us a little more about your bolt, carrier, key, rings, buffer, spring, and stock.

Everything, except for the gas block and handguard, is standard as supplied in kit from M&A Parts.  They state "minor parts included are from current US Govt contract machine shops."

The standard CAR 4 position stock was included but I have replaced that with the CAA collapsable stock.  I actually got a pretty good deal on the stock on eBay.  The auction was for the adustable stock with adjustable cheek piece and I won it for only $36 plus shipping.  I just swapped the CAA stock for the standard so the buffer tube is still that supplied with my kit.

I really like the CAA.  The adjustable cheek piece is nice but it gets in the way of the charging handle if the stock is collapsed.  I may end up selling the cheek piece.

My buffer and spring are, again, as standard in the carbine kit.

Standard bolt with 3 gas rings, carrier, and key.  The key has not been staked but the bolts remain tight.  I will be staking the key.  

Dano523 stated with a " .0035 difference between the gas tube and gas block tube channel, you going to have a hell of a lot of leaking".  The ID of the key is about 0.182" - 0.183" (it hard to get the calipers in for an accurate measurement) and the OD of the gas tube at the mushroom is 0.179".  Could this 0.003" - 0.004" difference cause an issue with cycling as well?  The mushroom is not worn, I only have 100 or so rounds through it.

I did contact Yankeed Hill about the opening in the gas block as Dano523 suggested.  They stated the opening should be 0.180" and mine measured 0.183".  They requested that I send it in for inspection to make sure it was in spec.  I shipped the gas block to them Saturday along with some of the photos in my original post to show  the carbon deposits one the gas tube.  Hopefully I will hear from YHM by the end of the week.  I will post an update once I find out about the gas block from Yankee Hill.  Oh, and by the way,  Yankee Hill's customer service has been great.
Link Posted: 2/17/2008 4:46:32 AM EDT
Best troubleshooting post ever.
Link Posted: 2/17/2008 6:12:31 PM EDT
I did receive a response from Yankee Hill.  They stated that the gas block was indeed in spec but would be sending me a new one.  I am still waiting on it.  Based on the photos of the gas tube I sent to YHM with my gas block, they questioned where the gas tube came from.  I informed them that it was new and included in the kit from M&A Parts.  I guess if the new gas block does not work, I will try replacing the gas tube, key, or possible changing to a different gas block if I continue to get carbon deposits on the tube.  
Link Posted: 2/18/2008 10:53:22 AM EDT
As a Newbie myself I found this thread very informative and I've never taken my AR apart other then field stripping for cleaning.  Great job on explaining things.
Link Posted: 2/18/2008 1:59:18 PM EDT
Great camera work!
Link Posted: 2/18/2008 2:06:24 PM EDT
This is a great wirte up of a problem and easy to see what the problem is and how to fix it.  I would be nice to have this tacked at the top as an example.

Link Posted: 2/29/2008 10:18:35 AM EDT
I got my new Specter gas block from Yankee Hill.  If you have any questions regarding their products, don't hesitate to contact them.  They offer great customer service.  Communication was great through both phone and email.  I checked the gas tube opening and it was tighter measuring 0.181" instead of 0.183".  I could feel a diffence when installing the gas tube.  I actually had to work the tube in.  

I finally had a chance to test fire  my AR with the new gas block.  It is still not functioning.  I fired about 20 rounds loaded in the mag one at a time.  The bolt is still not locking open and shells are not always ejecting.  Finally, I load 10 rounds in the mag and cycled them one at a time.  3 of the 10 shells ejected and landed at 3 o'clock about 2 feet away.  A couple of the shells were jammed against the barrel extension and the rest needed to be ejected manually.  A new round was never stripped from the mag unless cycled by hand.

I took everything apart again to examine the gas system.  Everything is lined up and tight.  There were still some carbon deposits on the tube by the gas block.  But based on other threads, this is common on new weapons and will eventually "seal" itself.


There was also quite a bit of deposits on the key end of the tube but I would guess this to be normal.  Am I wrong?


What's next???

  • Replace gas tube

  • Replace key

  • Cut down buffer spring

  • Shoot, shoot, shoot to break it in

Link Posted: 2/29/2008 12:19:16 PM EDT
Link Posted: 3/2/2008 3:14:53 PM EDT
Great writeup and pics.

+1 to everything the moderator said.

Do you notice any unusual resistance when cycling by hand?
Link Posted: 3/2/2008 5:31:51 PM EDT
Do you notice any unusual resistance when cycling by hand?

Cycling by had seems smooth.  However, as this is my first AR(not my first firearm), I have no basis of comparison.

Check the FCG for hammer wedging between the bottom of the carrier/top of the trigger/disco (at there rearward sections).

What kind of evidence would indicate wedging?  There doesn't appear to be any abnormal wear.

Link Posted: 3/2/2008 9:45:14 PM EDT
Link Posted: 3/3/2008 4:57:28 AM EDT
Disco = Disconnector, correct?

That "wear line" appears to be a result (or flaw) of the manufacturing process since it has the same finish in the depression as the rest of the part.  It does not look to me to be caused by contact with the hammer.


There are no markings on the back of the hammer to indicate contact.

Link Posted: 3/4/2008 6:15:14 PM EDT

Disco = Disconnector, correct?

That "wear line" appears to be a result (or flaw) of the manufacturing process since it has the same finish in the depression as the rest of the part.  It does not look to me to be caused by contact with the hammer.


There are no markings on the back of the hammer to indicate contact.


Stag disco?  Mine look like that too.
Link Posted: 3/4/2008 6:40:32 PM EDT
Stag disco?  Mine look like that too.

I don't know if it's Stag or not.  It came in the kit from M&A Parts.
Link Posted: 3/5/2008 4:48:10 PM EDT
Flow test the key on the carrier to confirm the the key is not leaking and the key port is aligned with the carrier port. doing the same with the gas tube/gas block and barrel is done at the same time.

What is the best (proper) way to perform a flow test on the key and gas block?
Link Posted: 3/5/2008 7:52:50 PM EDT
Link Posted: 3/6/2008 6:59:49 PM EDT
The parts were probably just wiped down with a little CLP.  I don't remember going all out with brushes, swabs, and picks.  I did try to make the parts look nice for the photo. Why do you ask?
Link Posted: 3/10/2008 9:05:35 PM EDT
Is it possible your magazine is sitting too high up in the well and the compressed rounds are placing too much pressure on the bolt carrier to cycle properly?

Heres a long shot, but does your CAA buttstock have a hole to let the buffer tube vent?  I know the hole is now days considerred a drain hole, but long ago people used to recommend NOT blocking the hole so less air is compressed by the buffer.  Before I get flamed too badly over this, please refer to  http://www.ak47.net/lite/topic.html?b=3&f=66&t=210213  under "exterior examination" item 3.  I doubt this is the problem but may as well eliminate all the variables.
Link Posted: 3/12/2008 3:13:20 AM EDT
Is it possible your magazine is sitting too high up in the well and the compressed rounds are placing too much pressure on the bolt carrier to cycle properly?

The majority of the rounds fired have been loaded one at a time in the magazine and fired.
Heres a long shot, but does your CAA buttstock have a hole to let the buffer tube vent?... I doubt this is the problem but may as well eliminate all the variables.

The CAA buttstock does not have a vent hole except for a 1/4" hole above the locking pin, on the top side of the tube.  The buffer tube covers this hole even when the stock is fully extended.  I removed the tube, covered the hole on top, and blew into it.  It seemed pretty air tight except for some air leaking out of the locking mechanism.  There is some play in the stock on the tube so I would guess that the stock is not really air tight when installed.  Initially, when I started this thread, the gun was tested with a standard carbine stock with an opening the size of the buffer tube prior to purchasing the CAA stock .  Some tweaking was done at the same time as replacing the stock so I will try the standard carbine stock next time I have a chance to shoot to "eliminate all the variables."  If this lack of air flow is contributing to the problem, you can be sure that the next step is to drill a vent hole in the CAA stock.
Link Posted: 3/12/2008 7:38:45 AM EDT
A few other thoughts:  check your bolt locking lugs for burrs, you have fairly substantial parkerizing wear on the locking lugs for 120ish rounds.  Be sure to lubricate your rifle very well during break in, there are a lot of friction points that need to seat.

Is the gas tube properly centerred in the bolt key openning? With the bolt head removed move the bolt carrier forward gently by hand and try to check the gas tube/bolt key alignment.  On my first barrel install 25 years ago I had a gas tube not lined up quite right, I took a punch that was approx the same size as the gas tube i.d. and gently aligned it.  The picture you posted of the gas key almost makes it look like the tube is contacting heavily on the 7 o'clock position facing the key openning, but it just might be the angle and the flash.

My final thought is that you should try some several different types of 5.56 ammo.  .223 doesnt generate as much pressure as 5.56 and could be problematic on a tight new rifle.  I believe I've read that .223 produces about 50,000 c.u.p. where 5.56 typically is around 60,000 c.u.p., but I could be off on those numbers.

Good luck on your next test firing and keep us updated please.
Link Posted: 3/25/2008 9:18:35 AM EDT
This would be a little work but it will tell a LOT. Fire one round with the gas block removed. Then try to extract the empty round by hand. If it's really hard to remove, you have a chamber problem. You could first try lubing a few rounds and see if they cycle. I'm also from Ohio. If you would, please email me. Your forum email is not enabled. I would like to ask a few questions about your rifle as I'm going to build one. If you try removing the gas block, stay clear of the barrel hole and wear a face shield or hold the gun in a device of some sort. Old tires and string are great for dangerous tests.

Good luck,

[email protected]
Link Posted: 4/4/2008 9:00:11 AM EDT
Here are two other things you can try, one cheap and simple, one a little more complicated and expensive:

1) After chambering a round, remove the magazine completely and try firing with an empty mag well.  One of my first AR builds (on an old cast EA lower) had a bolt hold open lever that was held slightly out of spec by the roll pin, and the top of the bolt hold open created drag on the bottom of the hammer channel in the bolt carrier.  Alternatively, remove the bolt catch and try firing with the magazine in place.

2) Switch to a full length buffer tube and stock.  This is really easy if you happen to have a shooting buddy with a full sized AR who will let you borrow it.  I was having trouble with my last 16 inch build I assembled with a telescoping stock- short stroking, failure to lock open after last round, inconsistent extractiion/ ejection- and all of it ceased when I put the 16 inch upper on my full length stock lower.  If this remedies the problem, you may be able to get normal function with the tele stock by using a heavier ("H") buffer.

Good luck.
Link Posted: 5/8/2008 3:33:19 PM EDT

resolved?  what was it?


Link Posted: 5/8/2008 4:24:58 PM EDT

Great camera work!

Agreed. Great write up. Very informative!
Link Posted: 5/14/2008 11:42:53 AM EDT
Actually the problem is still not resolved.  I was hoping to have this build completed before the birth of my first child.  Well... she is seven weeks old now.  I haven't had much free time on my hands a new parent.  I know this is not the solution to the problem, but for the heck of it, I tried clipping some of the coils off of the recoil spring.  I clipped the coils one at a time until it would cycle.  I did finally get it to cycle new rounds from the magazine.  It would lock back on empty but not all the way.  The thing is, I had to clip nearly 1/3  of the recoil spring off (cut to about 7.75") to get to this stage.  This is only about an inch or so longer than the buffer tube.  Anyhow, I have purchased and installed a new spring along with purchasing some 5.56 Ball ammo to try.  I just haven't had a chance to make it to the range.
Link Posted: 5/14/2008 11:22:32 PM EDT
I'm dissapointed on Dano's handling of this while I was gone...

Open up the gas port to 0.082"
It's way too small.
Link Posted: 5/15/2008 7:33:41 PM EDT
Link Posted: 6/3/2008 6:11:53 PM EDT
Ball ammo still did not cycle.  I ran a 20 round box through(cycling manually).  Maybe 1/4 of them ejected with no new round chambered.  I had 2 or 3 that were pulled from the chamber about 3/4" and required a bit of force to manually extract.  My friend watching said that the bolt moved back about an inch or so then moved forward re-chambering the fired round.

I am just about to the point of accepting defeat on this one.  I don't really have as much time to tinker as I used to since I have become a parent.  Honestly, it has become more like work and is no longer fun.  It has also become hard for me to justify having a near $1000 firearm that does not function when my Beretta 391 sporting clays shotgun, that gets weekly use, chews though 1000s of rounds without a hiccup and no more maintenance than an occasional wipe down.  Yes, I know they are two completely different systems that can hardly be compared.  However, if just one person reads this post and thinks to himself, "Maybe the AR-15 system is more than something that can just get slapped together and might be a little more than I think I can handle" or helps someone resolve a similar issue, I shall consider my endeavor, and this post, a victory.

Thank you all for you help and suggestions,

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