make sure your gas block is aligned correctly --- needs a little space behind it and the shoulder on the barrel--best way to check is to remove the GB to view the witness mark on the barrel, make sure it is centered around the Gas port

check to make sure your mag lips and spring are presenting the rounds correctly,they should nose up when in the mag

scan this thread for lots of helpful info too : http://68forums.com/forums/showthread.php?57071-Hints-Tips-and-general-information

 

What exactly is it doing that you call short stroking? If you load one round and fire will it lock back on the empty mag? Have you adjusted the gas block as far open as possible? May seem like silly questions but I was always taught to assume nothing, especially when trying to diagnose a problem.

 

May seem like silly questions but I was always taught to assume nothing, especially when trying to diagnose a problem.

Best advice.

 

nedro: Be patient, all of these bugs will eventually work themselves out. Its just part of building your own AR-15, regardless of chambering.

- Let's start with the ammunition, is it factory or handloads, which bullet, powder charge, etc? Some factory ammo is weaker than others.
- Also, if the gas block is of the set screw type, you can use the rear screw to properly locate the block by letting the set screw nestle down into the little dimple ARP puts exactly opposite the gas port.
- At the range did you incrementally adjust the gas block to more and more open positions until a fired round locked the bolt all the way back behind an empty magazine? This is the normal method to calibrate gas pressure, an adjustable gas block is much preferred over the non-adjustable types.
- Next, check your magazines to ensure the feed lips are pointing the bullet tips slightly upward on both sides so they don't jam into the feed ramps upon chambering. You can bend the lips slightly to adjust this factor, see instructions at the link below. Good luck and keep us informed about your fault isolation efforts. - CW

http://68forums.com/forums/showthread.php?7265-6-8-SPC-Magazine-Lip-Tweaking-Document

 

1 round without a mag will short cycle (not lock back) and will be a toss up whether or not you will find the empty shell in the chamber. I have not seen a broken rim on any of the rounds.

This is my third AR build. My other two are AR-15s.

Ammo: Remington 115g UMC.
In reading it seems that you guys say it's under powered. But muzzle velocity and energy is greater than Hornady, which seems to have some of the lowest reported muzzle velocity and energy of almost all of them. And yet they seem to be recommended as hot rounds.
A little education on my part would be appreciated.
I have been accumulating reloading equipment for the 6.8 and will eventually be asking questions in the reloading threads.
What are the commonly accepted Best Factory loads for an ARP build?

Gas block: has set screws. I had it opened up all the way to start. It was still open all the way when I installed the stock carbine spring and buffer. It went from 9 S/C per 10 to 6 S/C per ten.
I then read about over pressure so I started closing it down with no improvement.
My guess is that there is leakage due to the set screws for the orifice. I may try a non-adjustable one if everything checks out and no obstructions are found with the current set-up.

Mags: Thanks for the tips on the mags. I know that 1911s have a similar issue. the older mags are actually tapered. The newer mags simply cut of the last bit of retainer which will sometimes cause the round to point up so severely that it gets jammed at an angle between the slide and feed ramp.
I will check out the link.
It looks like I have allot to check.
Can someone recommend good factory rounds to use while trying to sort all of this out?
It would be much appreciated.
Thanks.

 

I'm calling bad extractor spring... add a o ring and that should get it to extract every time.... since you said brass may or may not be left in chamber leads me to believe its the extractor spring. and hand cycle the gun empty a few hundred times

 

Remington's 6.8 ammo has a reputation for being under powdered. Try some of the other ammo listed since it may be just an ammo issue. I've never had factory ammo cause problems in my ARP barrels or any other brands I have but I've never used Remington's ammo either. I would try some S&B FMJ if you can find it or Hornady 120 gr SST. But ruling out a problem with the gas block, gas system or adjustment of the block would be my first step. You may want to pull the block and check the carbon ring on the barrel at the gas port just to make sure the block was lined up correctly. Usually not hard to line up a block with ARP barrels due to the dimple in the barrel. But it happens.

 

As for Remington ammo velocities vs Hornady, it all comes down to what the factory states. Some test in 20 to 24 inch barrels and others test in 16 inch barrels. I know that shooting the same ammo in a 16 inch barrel vs a 20 inch barrel can give as much as 200 fps increase in the 20 inch as that is what I get. Most times the difference is between 100 and 150 fps difference between 16 and 20 inch barrels.

Hornady uses a 16 inch barrel for velocity testing and states it on the box along with their site. Remington gives a higher velocity but does not say what barrel length they use and many people have found that published velocities are frequently much less than actual velocities.

 

I doubt it is the extractor, not likely to cause short stroking. Also if gas is leaking you should see soot. Please explain what this means;

It was still open all the way when I installed the stock carbine spring and buffer. It went from 9 S/C per 10 to 6 S/C per ten.

Not sure what you mean by S/C per 10. I've never seen that terminology before.

 

I doubt it is the extractor, not likely to cause short stroking. Also if gas is leaking you should see soot. Please explain what this means; Not sure what you mean by S/C per 10. I've never seen that terminology before.

I'm guessing Stuck/Case?

 

I had about 7 FTX out of 10 with an ARP bolt and barrel. Might be what he means. Could be one of those weak extractor springs, mine was the weakest extractor I have ever seen.

 

But my point is that wouldn't likely cause short stroking/failure to lock back on an empty mag.

 

Sorry about the "S/C". It was my attempt at being lazy and not writing out Short Cycling.
The reason I stated that there are no broken rims was to let you know that the round has never been stuck in the chamber. It will always just come right out with no effort by manually cycling the bolt.
Sometimes the bolt doesn't go far enough back to extract the round, sometimes it does. And every once in a while, it cycles far enough back to load the next round.
I believe this is a low gas pressure problem that I need to closely look at.
My plans are to get some Hornady and federal to eliminate the ammo.
I buy on-line, so while waiting I will go though the gas system to check everything that Texas Hog Man has outlined.
I will also take a close look at the mags I have and go through the Mag link provided in this thread.
I'm going to keep the adjustable gas block on it for now and seal up all of the connecting parts of the gas system, and go from there.
At this point, I think I will hold off on any feed ramp operations until I get the gas system figured out.

Is there anything I am forgetting?
Thanks.

 

Short stroking is usually due to lack of gas port pressure. Could be the gas block is misaligned, a bad gas tube, or weak ammo. Gas pressure as related to ammo depends on how much gas the burning powder generates. The more gas a powder generates, the more pressure you have. You can easily check gas pressure by seeing where the ejected case falls. If it ejects about 4 o'clock give or take, then gas pressure is fine. But you can still have short stroking issues that are mechanical in nature. The Hornady 110 HPBT w/c factory load is a good one for testing function as well as accuracy.

It has been my observation over the last several years that most of the problems people have with functional issues are due to the carbine gas system. There are too many combinations of buffers, springs, stocks, gas blocks, etc., and the line between dead reliable functioning and not is very narrow.

First, find some good ammo and see where it ejects to eliminate the gas system as a problem. Make sure you aren't retarding the bolt carrier due to friction. Run it wet so eliminate lubrication issues. Make sure the mags are not locking up too high in the receiver and that the follower is presenting rounds properly to the feed ramps.

Every single AR I've built uses mid-gas, and standard A2 stock and buffers, and not a single adjustable gas block in the bunch. The most recent one that I tested for the first time Friday is a wildcat .358 Yeti. I was the only mid-gas barrel in the initial batch, and the gas port was the same as what they drilled on the carbine gas barrels. I had to drill it out, and didn't know if it would be enough (or maybe too much), going from .07x to .100". But, the first test round fired kicked out perfectly about six feet at 4 o'clock. My point is that even with experimental cartridges and a field fix to the gas port, the mid-gas/A2 setup just works. When you start getting fancy with buffers, stocks, gas blocks, etc., that's when things start going squirrely. Something to think about on future builds.

 

i recently had a bolt where the front edge of the extractor sat further back than any of my other bolts, the rifle was a single shot from day one, new bolt with the front edge of the extractor closer to the front edge of the bolt solved that problem. this original extractor (by eyeball) was over a 32nd of inch from the edge of the bolt. take the bolt and stand it upright on it's lugs, check the gap between the extractor and another bolt if you have one there. the one i had problems with was short enough that i don't think it ever engaged the case rim

 

I usually defer to all the guys who have already posted, but my question would be could gas rings where the gap is aligned also cause the same issue as the gas is bypassing the bolt and not enough is exerting force to make it travel to the rear? I ask because it sounds like he has done a lot of troubleshooting on the gas block.

 

Good suggestion. I'll go through everything I can think of and get back.
But not tonight. Tonight is for the kids.

 

Check the gas carrier key on top of the bolt. Make sure it's not leaking gas. My first AR had a gas carrier key where the opening the gas tube fits into was out of round and allowed gas to escape. Also, make sure your gas tube is installed correctly. If that checks out, I like the idea of trying different ammo first then replacing the gas block to see how it runs.

Regarding the rings, I've read where it doesn't matter whether they are gapped or spaced correctly. If you think about it, it makes sense consider how the rings are moved around as the bolt flies back and forth. I think the rings are compressed when the bolt is forced back. And I've read, the pressure of the bolt face is enough to cycle the AR. I really don't know whether that true and would welcome someone to comment that has more experience on it. I've been shooting ARs since the early 1990's and I've never had to replace the gas rings on any of my ARs. Nor have I had one fail to cycle due to gas ring failure that I'm aware of.

 

Many ballistics claims are exaggerated. The #1 market for unusually long barrels must be to reloading suppliers, for example. So don't feel alone in believing what you read.

On the other hand, I don't know why ARP's 6.8 barrels have to have two different gas port size options. I don't see it with other AR rifles. Buy an AR anywhere else and it is expected to work without matching the ammo to the gas port. Even with crappy UMC ammo.

 

Many ballistics claims are exaggerated. The #1 market for unusually long barrels must be to reloading suppliers, for example. So don't feel alone in believing what you read.

On the other hand, I don't know why ARP's 6.8 barrels have to have two different gas port size options. I don't see it with other AR rifles. Buy an AR anywhere else and it is expected to work without matching the ammo to the gas port.

It is because I don't want the barrels over-gassed with full pressure ammo. If the carrier starts moving while the bullet is in the bore it hurts accuracy.
All those other guns you talk about need H2 buffers and big spring to slow the carrier down...why the H2 buffers and adjustable gas blocks were developed...over-gassed guns.

 

I agree.
I do not want to patch up the problem with parts that will compensate for my problem.
I want to get it properly gassed and will use the appropriate ammo pressures as I begin to work up my loads.
I got all fancy with the build and am now needing to cycle it back a bit. (pun intended)

 

n: IMO, handloading your own ammo avoids all the ills of factory-made stuff. It is especially useful in finding that ideal combo of bullet and powder that your barrel really does best with. good luck - CW

 

OK, just an update.
I took the rifle apart to look at the gas block. The hole in the barrel was exactly centered on the gas bock port. A micrometer couldn't have centered it better.
There was very minor escaped gas at both the adjusting screw and the split pin that holds the gas tube and gas block together.
I bought a new, Made in the USA, standard low profile gas block and installed it.
I have ordered the following ammo to test in my rifle: Hornady AG 110gr BTHP WC, Federal AE 115gr FMJ, Hornady 120gr SST, and Federal Fusion 90gr SP.
I have not done anything to the mags yet.
As I understand it, the rounds need to sit in the mags with the tips up at about a 20 degree angle.
I'll take them all apart and do a thorough clean and polish. Then if need be, taper the mag lips.
Anything else anyone can think of?
Thanks.

 

nedro: With the lips properly tweaked, the bullet tips should point upward only about 3/16" to 1/4" above the top edge of the magazine. 20 degrees would be way too much, the rounds would not stay put at that angle. A little bit of metal bending is sufficient. In my experience, too much bending back and forth and your mags are trashed. - CW

 

You have a classic under gassing problem. The good thing is you can just open up the port as previously mentioned if you want to run the Remington ammo, as yer using an adjustable gas block. I find that Remington ammo is very accurate in most of my guns.

Also, possibly, an extractor spring problem. Might as well throw an o-ring or a new extractor spring on there. Sometimes I can't get o-rings to fit.

Good news is...yer on it and it's all easy to fix.

 

I had a short stoke issue on 2 builds recently. After diagnosing both guns it was an out of spec gas tube on both rigs. Changed the tubes on both guns and they run like a champ! Lesson learned was don't overlook the small things.

 

What aspect of the tube was 'out of spec'?

 

A couple of years ago, Owenslee posted a thread concerning gas tubes which was very enlightening. He found the holes in tubes being smaller than spec. As I recall, he drilled them out to bring them back into spec.

 

It's all ready to go.
Hole was lined up between block and tube on the adjustable block and the new one I just installed.
I got the ammo I ordered and will probably go out this weekend to try it all out.
Again, I want to be clear about this. I did not build the running assembly as ARP recommended and also used the one cartridge that they specifically said not to use with their stock barrel.
I'll let you guys know how it goes.
I appreciate all the help and suggestions. I'm new to this platform and will be working up my own loads very soon.

 

Well,
I got to go out to the range yesterday for a short while.
Before I went, I tweeked a mag so the rounds wouldn't crash into the feed ramps and get damaged.
It seemed to work out as there were no issues for the first 10 rounds.
I was using Hornady 120g SST and used the first 10 rounds to sight in the scope that I took off to move back a bit for a better sight pic.
I went out to reset the target and came back to the bench. I loaded up the other 10 rounds from the box I had opened up into the same mag, released the charging handle and the bolt could not go into battery, not even with some good forward assist bumps.
Then I had a bear of a time pulling the bolt back. I had to pry it out from the bottom after separating the halves.
I took a good look at the round and saw nothing wrong with it, so I put it to the side and charged the next round.
Same thing. Jammed solid.
When the round gets freed up (pulled out of the chamber) both were jammed into the bolt so hard that they just stayed straight. I had to pop them out to the side to eject them from the bolt.
The rain was coming so I loaded up my stuff and went home after ten rounds.
When I got home, I took a really good look at everything and couldn't find anything in the chamber or around the bolt that would've changed things so much that the gun wouldn't go into battery.
I cleaned everything up on the bolt and could not believe how much crud was collected on it from just about 60 rounds through it from brand new.
I didn't think about trying different ammo since the first ten shots were perfect. I even ended up running shot 9 and 10 through one hole (They were touching) within a 1/2 MOA from bullseye.I was very happy when I retested the ammo. It went in no problem. Just like it should. I ejected the round and the next one jammed. Just like the others.
I never thought to try the different ammo I had, since my first ten shorts were flawless. No feeding, cycling or firing issues. And I loaded the next ten from the same box as the first ten.
Well, this was when I started trying out everything I had, and wouldn't you know it!
9 out of the next ten Hornady rounds would not go into battery. I couldn't find a Federal or Remington round that would not go into battery. All of them were good. The Hornady American Gunner will also jam.
So I measured the cartridges. And sure enough, the Hornadys were 0.002" to 0.004" longer than the Federal or Remington brass.
I was measuring from base to tip of the brass (where the cantulure is on the bullet).
So in a nut shell, my headspace is too small. DAMNIT!

 

I was measuring from base to tip of the brass (where the cantulure is on the bullet).
So in a nut shell, my headspace is too small. DAMNIT!

Hornady has been known for having out of spec loaded ammo.

That is not how you measure/check headspace and doesn't mean your chamber is out of spec.

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Ya, I know that.
It's just really hard to get a good measurement at the shoulder with a set of calipers.
I can get gauges for about $75. But I have a feeling it's just on the tight side. Another reason to reload.

 

"When the round gets freed up (pulled out of the chamber) both were jammed into the bolt so hard that they just stayed straight. I had to pop them out to the side to eject them from the bolt."
That should not happen.

 

the bolt was jamming between the base of the cartridge and the back of the cogs.

 

well,
just ordered a set of go, no-go gauges.
Not really sure why though. I can't flippin fix it with the tools I own.
If nothing else, I suppose you all could get a good laugh at my situation. I know I am.

 

Try new bolt or complete bcg

 

I bought the ARP carrier and super bolt in NiB.
And I have a question for anyone who has also bought this combo.
Does it look like polished steel?
mine came to me like that. It looks awesome, but I thought it would have some sort of coating on it.
I'm just trying to confirm that I didn't get an unfinished BCG.

 

NiB can appear anywhere from chrome like to a tarnished silver.

I seriously doubt Harrison sent you an assembled bolt that wasn't heat treated or NiB treated.

Here are a couple 6.8 bolts, one being an ARP Superbolt.

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